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sources.²⁵ From this base of experience, the fingerprint community asserts

that the latent print examiner learns to judge whether there is sufficient

detail (which varies with image quality) to make a source determination

during the evaluation phase of ACE-V.

The latent print community in the United States has eschewed numeri-

cal scores and corresponding thresholds, because those developed to date²⁶

have been based only on minutia, not on the unique features of the fric-

tion ridge skin (e.g., lengths of ridges, shapes of ridges, crease lengths and

shapes, scar lengths and shapes). Additionally, thresholds based on count-

ing the number of features that correspond, lauded by some as being more

“objective,” are still based on primarily subjective criteria—an examiner

must have the visual expertise to discern the features (most important in

low-clarity prints) and must determine that they are indeed in agreement.

A simple point count is insufficient for characterizing the detail present in

a latent print; more nuanced criteria are needed, and, in fact, likely can be

determined.

Reporting of Results

SWGFAST has promulgated three acceptable conclusions resulting from

latent print comparison: individualization (or identification), exclusion, or

inconclusive.²⁷ Although adherence to this standard is common, some latent

print examiners report either “identification” or “negative” results. “Nega-

tive” (or sometimes “not identified”) is an ambiguous conclusion, and it

could mean excluded, inconclusive, or unable to locate after exhaustive

search. It is problematic that the meaning of “negative” may be specific to

a particular agency, examiner, or case.

Latent print examiners report an individualization when they are con-

fident that two different sources could not have produced impressions with

the same degree of agreement among details. This is a subjective assessment.

There has been discussion regarding the use of statistics to assign match

probabilities based on population distributions of certain friction ridge

features. Current published statistical models, however, have not matured

past counts of corresponding minutia and have not taken clarity into con-

sideration. (This area is ripe for additional research.) As a result, the friction

ridge community actively discourages its members from testifying in terms

of the probability of a match; when a latent print examiner testifies that two

²⁵ T. Busey and J. Vanderkolk. 2005. Behavioral and electrophysiological evidence for con-

figural processing in fingerprint experts. Vision Research 45:431-448.

²⁶ See, e.g., I.W. Evett and R.A. Williams. 1996. A review of the sixteen points fingerprint

standard in England and Wales. Journal of Forensic Identification 46(1):49-73.

SWGFAST. 2002. Friction Ridge Examination Methodology for Latent Print Examiners.

Available at www.swgfast.org/Training_to_Competency_for_Latent_Print_Examiners_2.1.pdf.

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impressions “match,” they are communicating the notion that the prints

could not possibly have come from two different individuals.

As noted in Chapter 3, Jennifer Mnookin of the University of Califor-

nia, Los Angeles School of Law summarized the reporting of fingerprint

analyses as follows:

At present, fingerprint examiners typically testify in the language of ab-

solute certainty. Both the conceptual foundations and the professional

norms of latent fingerprinting prohibit experts from testifying to identifica-

tion unless they believe themselves certain that they have made a correct

match. Experts therefore make only what they term ‘positive’ or ‘absolute’

identifications—essentially making the claim that they have matched the

latent print to the one and only person in the entire world whose fingertip

could have produced it . . . Given the general lack of validity testing for

fingerprinting; the relative dearth of difficult proficiency tests; the lack of

a statistically valid model of fingerprinting; and the lack of validated stan-

dards for declaring a match, such claims of absolute, certain confidence in

identification are unjustified . . . Therefore, in order to pass scrutiny under

Daubert, fingerprint identification experts should exhibit a greater degree

of epistemological humility. Claims of ‘absolute’ and ‘positive’ identifica-

tion should be replaced by more modest claims about the meaning and

significance of a ‘match.’²⁸

Summary Assessment

Historically, friction ridge analysis has served as a valuable tool, both

to identify the guilty and to exclude the innocent. Because of the amount of

detail available in friction ridges, it seems plausible that a careful compari-

son of two impressions can accurately discern whether or not they had a

common source. Although there is limited information about the accuracy

and reliability of friction ridge analyses, claims that these analyses have zero

error rates are not scientifically plausible.

ACE-V provides a broadly stated framework for conducting friction

ridge analyses. However, this framework is not specific enough to qualify as

a validated method for this type of analysis. ACE-V does not guard against

bias; is too broad to ensure repeatability and transparency; and does not

guarantee that two analysts following it will obtain the same results. For

these reasons, merely following the steps of ACE-V does not imply that one

is proceeding in a scientific manner or producing reliable results. A recent

²⁸ J.L. Mnookin. 2008. The validity of latent fingerprint identification: Confessions of

a fingerprinting moderate. Law, Probability and Risk 7:127. See also the discussion in C.

Champod. 2008. Fingerprint examination: Towards more transparency. Law Probability and

Risk 7:111-118.

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143

paper by Haber and Haber²⁹ presents a thorough analysis of the ACE-V

method and its scientific validity. Their conclusion is unambiguous: “We

have reviewed available scientific evidence of the validity of the ACE-V

method and found none.”³⁰ Further, they state:

[W]e report a range of existing evidence that suggests that examiners differ

at each stage of the method in the conclusions they reach. To the extent

that they differ, some conclusions are invalid. We have analysed the ACE-

V method itself, as it is described in the literature. We found that these

descriptions differ, no single protocol has been officially accepted by the

profession and the standards upon which the method’s conclusions rest

have not been specified quantitatively. As a consequence, at this time the

validity of the ACE-V method cannot be tested.³¹

Recent legal challenges, New Hampshire vs. Richard Langill³² and

Maryland vs. Bryan Rose,³³ have also highlighted two important issues for

the latent print community: documentation and error rate. Better documen-

tation is needed of each step in the ACE-V process or its equivalent. At the

very least, sufficient documentation is needed to reconstruct the analysis,

if necessary. By documenting the relevant information gathered during the

analysis, evaluation, and comparison of latent prints and the basis for the

conclusion (identification, exclusion, or inconclusive), the examiner will

create a transparent record of the method and thereby provide the courts

with additional information on which to assess the reliability of the method

for a specific case. Currently, there is no requirement for examiners to

document which features within a latent print support their reasoning and

conclusions.

Error rate is a much more difficult challenge. Errors can occur with

any judgment-based method, especially when the factors that lead to the

ultimate judgment are not documented. Some in the latent print community

argue that the method itself, if followed correctly (i.e., by well-trained ex-

aminers properly using the method), has a zero error rate. Clearly, this as-

sertion is unrealistic, and, moreover, it does not lead to a process of method

improvement. The method, and the performance of those who use it, are

inextricably linked, and both involve multiple sources of error (e.g., errors

in executing the process steps, as well as errors in human judgment).

Some scientific evidence supports the presumption that friction ridge

patterns are unique to each person and persist unchanged throughout a

²⁹ Mnookin, op. cit.

³⁰ Ibid., p. 19.

³¹ Ibid.

³² 157 N.H. 77, 945 A.2d 1 (N.H., April 04, 2008).

³³ No. K06-0545 (MD Cir. Ct. Oct. 19, 2007).

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STRENGTHENING FORENSIC SCIENCE IN THE UNITED STATES

lifetime.³⁴ Uniqueness and persistence are necessary conditions for friction

ridge identification to be feasible, but those conditions do not imply that

anyone can reliably discern whether or not two friction ridge impressions

were made by the same person. Uniqueness does not guarantee that prints

from two different people are always sufficiently different that they cannot

be confused, or that two impressions made by the same finger will also be

sufficiently similar to be discerned as coming from the same source. The

impression left by a given finger will differ every time, because of inevi-

table variations in pressure, which change the degree of contact between

each part of the ridge structure and the impression medium. None of these

variabilities—of features across a population of fingers or of repeated im-

pressions left by the same finger—has been characterized, quantified, or

compared.³⁵

To properly underpin the process of friction ridge identification, ad-

ditional research is also needed into ridge flow and crease pattern distribu-

tions on the hands and feet. This information could be used to limit the

possible donor population of a particular print in a statistical approach

and could provide examiners with a more robust understanding of the

prevalence of different ridge flows and crease patterns. Additionally, more

research is needed regarding the discriminating value of the various ridge

formations and clusters of ridge formations.³⁶ This would provide examin-

ers with a solid basis for the intuitive knowledge they have gained through

experience and provide an excellent training tool. It also would lead to a

good framework for future statistical models and provide the courts with

additional information to consider when evaluating the reliability of the

science. Recently, research has begun to build some of this basis.³⁷

³⁴ F. Galton. 1892. Fingerprints. New York: MacMillan; H. Cummins and C. Midlo. 1943.

Finger Prints, Palms and Soles: An Introduction of Dermatoglyphics. Philadelphia: The Blakis-

ton Company; A. Hale. 1952. Morphogenesis of volar skin in the human fetus. The American

Journal of Anatomy 91:147-173; S. Holt and L.S. Penrose. 1968. The Genetics of Dermal

Ridges. Springfield, IL: Charles C Thomas Publishing; W. Montagna and P. Parakkal. 1974.

The Structure and Function of Skin. New York: Academic Press; J. Raser and E. O’Shea. 2005.

Noise in gene expression: Origins, consequences, and control. Science 39:2010-2013.

³⁵ Some in the friction ridge community point to an unpublished 1999 study by the Lock-

heed-Martin Corporation, the “50K vs. 50K Fingerprint Comparison Test,” as evidence of

the scientific validity of fingerprint “matchup.” But that study has several major design and

analysis flaws, as pointed out in D.H. Kaye. 2003. Questioning a courtroom proof of the

uniqueness of fingerprints. International Statistical Review 71(3):524. Moreover, even if it

were valid, the study provides only a highly optimistic estimate of the reliability of friction

ridge analyses, biased toward highly favorable conditions.

³⁶ Haber and Haber also provide a sensible research agenda for enhancing the validity of

fingerprint comparisons.

³⁷ E.g., C. Neumann, C. Champod, R. Puch-Solis, N. Egli, A. Anthonioz, and A. Bromage-

Griffiths. 2007. Computation of likelihood ratios in fingerprint identification for configu-

rations of any number of minutiae. Journal of Forensic Sciences 52(1):54-64; N.M. Egli,

FORENSIC SCIENCE DISCIPLINES

145

There is also considerable room for research on the various factors that

affect the quality of latent prints (e.g., condition of the skin, residue, me-

chanics of touch). Formal research could provide examiners with additional

tools to support or refute distortion explanations. Currently, distortion and

quality issues are typically based on “common sense” explanations or on

information that is passed down through oral tradition from examiner to

examiner. A criticism of the latent print community is that the examiners

can too easily explain a “difference” as an “acceptable distortion” in order

to make an identification.³⁸

OTHER PATTERN/IMPRESSION EVIDENCE:

SHOEPRINTS AND TIRE TRACKS

Other pattern evidence, also referred to as impression evidence, occurs

when an object such as a shoe or a tire leaves an impression at the crime

scene or on another object or a person. Impressions can be either two

dimensional, such as shoeprints in dust, or three dimensional, such as tire

track impressions in mud. Shoeprints and tire tracks are common types of

impression evidence examined by forensic examiners, but the list of po-

tential types of impression evidence is long. Examples include bite marks,

markings on bullets and cartridge cases, ear prints, lip prints, toolmarks,

some bloodstain patterns, and glove prints.³⁹ Although there are general

approaches concerning the analytical sequence of various types of impres-

sion evidence, each has its own set of characteristics. For example, some

types of impression evidence, such as those arising from footwear and tires,

require knowledge of manufacturing and wear, while other types, such as

ear prints and bloodstain patterns, do not. Because footwear and tire track

impressions comprise the bulk of the examinations conducted, the remarks

in this section are specifically focused on these analyses. Bite marks, mark-

ings on bullets and cartridge cases, and bloodstain patterns are covered in

later sections in this chapter.

C. Champod, and P. Margot. 2007. Evidence evaluation in fingerprint comparison and auto-

mated fingerprint identification systems—Modelling within finger variability. Forensic Science

International 167(2-3):189-195.

³⁸ U.S. Department of Justice, Office of the Inspector General. 2006. A Review of the FBI’s

Handling of the Brandon Mayfield Case. Office of the Inspector General Oversight and Review

Division, January.

³⁹ M. Liukkonen, H. Majamaa, and J. Virtanen. 1996. The role and duties of the shoeprint/

toolmark examiner in forensic laboratories. Forensic Science International 82:99-108.

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STRENGTHENING FORENSIC SCIENCE IN THE UNITED STATES

Sample Data and Collection

Impression evidence at the scene is generally of two types: latent (invis-

ible to the naked eye) or patent (visible). The quality of impression evidence

left at the scene cannot be controlled, but failures in the initial scene work

used to collect, preserve, and possibly enhance the evidence will degrade the

quality of the evidence eventually used for comparative analysis. After doc-

umentation at the scene, the evidence is preserved and possibly enhanced

using techniques such as those based on chemistry (e.g., metal detection),

physical characteristics (e.g., super glue fuming, powder dusting, casting),

or transfer onto a contrasting surface (e.g., electrostatic transfer or gel lift-

ing). The quality of the enhanced impression that is used for comparison

will depend largely on the experience, training, and scientific knowledge of

the scene investigator as well as the agency’s resources.

Although some analysis of impression evidence might begin at the

scene, the comparison of scene evidence to known exemplars occurs in the

laboratory. The educational background of forensic scientists who examine

shoeprints and tire track impressions runs the gamut from a high school

diploma to scientists with Ph.D.s. Identifications are largely subjective and

are based on the examiner’s experience and on the number of individual,

identifying characteristics in common with a known standard.

Analyses

The goal of impression evidence analysis is to identify a specific source

of the impression, and the analytical process that this follows generally is

an accepted sequence: identifying the class (group) characteristics of the

evidence, followed by locating and comparing individual, identifying (also

termed accidental or random) characteristics.⁴⁰

Class characteristics of footwear and tires result from repetitive, con-

trolled processes that are typically mechanical, such as those used to manu-

facture items in quantity. Although defined similarly by various authors,

Bodziak describes footwear class characteristics as “an intentional or un-

avoidable characteristic that repeats during the manufacturing process and

is shared by one or more other shoes.”⁴¹ For tires, Nause defines class char-

acteristics as, “[p]hysical characteristics acquired during the manufacturing

process (made from the same mold) that tires have in common.”⁴² He con-

tinues, “Class characteristics can often be combined to limit a tire impres-

sion to a very select group within the overall group bearing similar class

⁴⁰ Ibid.

⁴¹ W.J. Bodziak. 1999. Footwear Impression Evidence-Detection, Recovery, and Examina-

tion. Boca Raton, FL: CRC Press, 2nd ed., p. 329.

⁴² Nause, op. cit.

FORENSIC SCIENCE DISCIPLINES

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characteristics. (In the field of forensic tire evidence, class characteristics

often refer to such things as design, pattern, size, shape, mold variations,

etc.).”⁴³ Regardless of the type of impression evidence, class characteristics

are not sufficient to conclude that any one particular shoe or tire made

the impression. That latter step—which is not always possible—requires

comparison of the individual identifying characteristics on the impression

evidence with those on a shoe or tire that is suspected of leaving the im-

pression. These individual characteristics occur during the normal use of

an item, sometimes called wear and tear,⁴⁴ and are created by “random,

uncontrolled processes.”⁴⁵ For footwear, Bodziak writes that “individual

identifying characteristics are characteristics that result when something is

randomly added to or taken away from a shoe outsole that either causes

or contributes to making that shoe outsole unique.”⁴⁶ Such characteristics

might include cuts, scratches, gouges, holes, or random inclusions that

result from manufacturing, such as bubbles, and those that result from

adherent substances, such as rocks, chewing gum, papers, or twigs.

Following analysis of the impression, an identification is determined or

ruled out according to the number of individual characteristics the evidence

has in common with the suspected source. But there is no defined threshold

that must be surpassed, nor are there any studies that associate the number

of matching characteristics with the probability that the impressions were

made by a common source. It is generally accepted that the specific number

of characteristics needed to assign a definite positive identification depends

on the quality and quantity of these accidental characteristics and the cri-

teria established by individual laboratories.⁴⁷ According to Cassidy, many

factors and accidental characteristics are required before a positive identi-

fication can be established; however, the most important are the examiner’s

experience, the clarity of the impression, and the uniqueness of the charac-

teristic.⁴⁸ Proficiency testing for examiners of impression evidence is avail-

able through Collaborative Testing Service, Inc., but the proficiency tests

for footwear impressions include samples that are either a match or not a

match⁴⁹—that is, none of the samples included in the tests have the sort of

ambiguities that would lead an experienced examiner to an “inconclusive”

⁴³ Ibid.

⁴⁴ M.J. Cassidy. 1980. Footwear Identification. Quebec, Canada: Government Printing

Office Centre.

⁴⁵ K. Inman and N. Rudin. 2001. Principles and Practice of Criminalistics. Boca Raton, FL:

CRC Press, p. 129.

⁴⁶ Ibid., p. 335.

⁴⁷ Liukkonen, Majamaa, and Virtanen, op. cit.

⁴⁸ Cassidy, op. cit.

⁴⁹ H. Majamaa and Y. Anja. 1996. Survey of the conclusions drawn of similar footwear

cases in various crime laboratories. Forensic Science International 82:109-120.

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conclusion. IAI has a certification program for footwear and tire track ex-

aminers.⁵⁰ The group’s recommended course of study has 13 segments, and

each segment includes a suggested reading list and practical and/or written

exercises. The student must pass an examination. This course of study does

not require an understanding of the scientific basis of the examinations,

and it does not recommend the use of a scientific method. Also, there is no

provision or recommendation for proficiency testing or continuing educa-

tion. SWGTREAD, a group of footwear and tire track examiners formed by

the FBI, recommends that a trainee candidate have (1) a bachelor’s degree

(preferably in a physical or natural science) from an accredited college or

university; or (2) an associate degree or 60 college semester hours, plus two

years of job-related forensic experience; or (3) a high school diploma or

equivalent, plus four years of job-related forensic experience.⁵¹

Scientific Interpretation and Reporting of Results

For footwear evidence, Fawcett⁵²and Bodziak⁵³ have attempted to

assign probabilistic or statistical significance to impression comparisons.

Generally, shoeprint and tire track examiners prefer nonstatistical language

to report or to testify to the result of their findings. Terms such as “positive

identification” and “nonidentification” can be used to indicate an identifica-

tion or nonidentification, respectively, and “nonconclusive” would indicate

situations in which the analysis falls short of either of the other two.⁵⁴

In a European survey, examiners were given identical mock cases. Ac-

cidental, identifying characteristics were purposely put onto the sole of

new shoes, and examiners were asked to make a statement concerning the

strength of matches. The results of the survey concluded that there were

considerable differences in the conclusions reached by different laboratories

examining identical cases.”⁵⁵ SWGTREAD recommends terminology such

as:

•

“identification” (definite conclusion of identity)

•

“probably made” (very high degree of association)

⁵⁰ Recommended Course of Study for Footwear & Tire Track Examiners. 1995. Mendota

Heights, MN: International Association for Identification.

⁵¹ SWGTread. 2006. Guide for Minimum Qualifications and Training for a Forensic

Footwear and/or Tire Tread Examiner. Available at www.theiai.org/guidelines/swgtread/

qualifications_final.pdf.

⁵² A.S. Fawcett. 1970. The role of the footmark examiner. Journal of the Forensic Science

Society 10:227-244.

⁵³ Bodziak, op. cit., pp. 342-346.

⁵⁴ Ibid.

⁵⁵ H. Majamaa and Y. Anja., op. cit.

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149

•

“could have made” (significant association of multiple class

characteristics)

•

“inconclusive” (limited association of some characteristics)

•

“probably did not make” (very high degree of nonassociation)

•

“elimination” (definite exclusion)

•

”unsuitable” (lacks sufficient detail for a meaningful comparison).

Additionally, SWGTREAD discourages the use of once common ter-

minology, such as “consistent with” (acceptable when used to describe

a similarity of characteristics), “match/no match,” “responsible for/not

responsible for,” and “caused with/not caused with.”⁵⁶ Neither the IAI nor

SWGTREAD address the statistical evaluation of impression evidence.

Summary Assessment

The scientific basis for the evaluation of impression evidence is that

mass-produced items (e.g., shoes, tires) pick up features of wear that,

over time, individualize them. However, because these features continue

to change as they are worn, elapsed time after a crime can undercut the

forensic scientist’s certainty. At the least, class characteristics can be identi-

fied, and with sufficiently distinctive patterns of wear, one might hope for

specific individualization. However, there is no consensus regarding the

number of individual characteristics needed to make a positive identifica-

tion, and the committee is not aware of any data about the variability of

class or individual characteristics or about the validity or reliability of the

method. Without such population studies, it is impossible to assess the

number of characteristics that must match in order to have any particular

degree of confidence about the source of the impression.

Experts in impression evidence will argue that they accumulate a sense

of those probabilities through experience, which may be true. However,

it is difficult to avoid biases in experience-based judgments, especially in

the absence of a feedback mechanism to correct an erroneous judgment.

These problems are exacerbated with the less common types of impression

evidence. For example, a European survey found that 42 laboratories con-

ducted 28,093 shoeprint examinations and 41 laboratories conducted 591

tire track examinations, but only 14 laboratories conducted a total of 21 lip

print examinations and 17 laboratories conducted a total of 100 ear print

examinations.⁵⁷ Although one might argue that those who perform the

⁵⁶ SWGTREAD. 2006. Standard Terminology for Expressing Conclusions of Forensic

Footwear and Tire Impression Examinations. Available at www.theiai.org/guidelines/swgtread/

terminology_final.pdf.

⁵⁷ Liukkonen, Majamaa, and Virtanen, op. cit.

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work in laboratories that conduct hundreds or thousands of evaluations of

impression evidence develop useful experience and judgment, it is difficult

to assert that the field has enough collective judgment about the variabilities

in lip prints and ear prints based on tens of examinations. The community

simply does not have enough data about the natural variability of those less

frequent impressions, absent the presence of a clear deformity or scar, to

infer whether the observed degree of similarity is significant.

Most of the research in the field is conducted in forensic laboratories,

with the results published in trade journals, such as the Journal of Forensic

Identification. With regard to reporting, SWGTREAD is moving toward the

use of standard language to convey the conclusions reached.⁵⁸ But neither

IAI nor SWGTREAD addresses the issue of what critical research should be

done or by whom, critical questions that should be addressed include the

persistence of individual characteristics, the rarity of certain characteristic

types, and the appropriate statistical standards to apply to the significance

of individual characteristics. Also, little if any research has been done to

address rare impression evidence. Much more research on these matters is

needed.

TOOLMARK AND FIREARMS IDENTIFICATION

Toolmarks are generated when a hard object (tool) comes into contact

with a relatively softer object. Such toolmarks may occur in the commis-

sion of a crime when an instrument such as a screwdriver, crowbar, or wire

cutter is used or when the internal parts of a firearm make contact with the

brass and lead that comprise ammunition. The marks left by an implement

such as a screwdriver or a firearm’s firing pin depend largely on the manu-

facturing processes—and manufacturing tools—used to create or shape it,

although other surface features (e.g., chips, gouges) might be introduced

through post-manufacturing wear. Manufacturing tools experience wear

and abrasion as they cut, scrape, and otherwise shape metal, giving rise

to the theory that any two manufactured products—even those produced

consecutively with the same manufacturing tools—will bear microscopically

different marks. Firearms and toolmark examiners believe that toolmarks

may be traced to the physical heterogeneities of an individual tool—that is,

that “individual characteristics” of toolmarks may be uniquely associated

with a specific tool or firearm and are reproduced by the use of that tool

and only that tool.

The manufacture and use of firearms produces an extensive set of

⁵⁸ SWGTREAD. 2006. Standard Terminology for Expressing Conclusions of Forensic

Footwear and Tire Impression Examinations. Available at www.theiai.org/guidelines/swgtread/

terminology_final.pdf.

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151

specialized toolmarks. Gun barrels typically are rifled to improve accuracy,

meaning that spiral grooves are cut into the barrel’s interior. The process

of cutting these grooves into the barrel leaves marks and scrapes on the

relatively softer metal of the barrel.⁵⁹ In turn, these markings are transferred

to the softer metal of a bullet as it exits the barrel. Over time, with repeated

use (and metal-to-metal scraping), the marks on a barrel (and the corre-

sponding “stria” imparted to bullets) may change as individual imperfec-

tions are formed or as cleanliness of the barrel changes. The brass exterior

of cartridge cases receive analogous toolmarks during the process of gun

firing: the firing pin dents the soft primer surface at the base of the cartridge

to commence firing, the primer area is forced backward by the buildup of

gas pressure (so that the texture of the gun’s breech face is impressed on

the cartridge), and extractors and ejectors leave marks as they expel used

cartridges and cycle in new ammunition.

Firearms examination is one of the more common functions of crime

laboratories. Even small laboratories with limited services often perform

firearms analysis. In addition to the analysis of marks on bullets and car-

tridges, firearms examination also includes the determination of the firing

distance, the operability of a weapon, and sometimes the analysis of primer

residue to determine whether someone recently handled a weapon. These

broader aspects are not covered here.

Sample and Data Collection

When a tool is used in a crime, the object that contains the tool marks

is recovered when possible. If a toolmark cannot be recovered, it can be

photographed and cast. Test marks made by recovered tools can be made

in a laboratory and compared with crime scene toolmarks.

In the early 1990s, the FBI and the Bureau of Alcohol, Tobacco, Fire-

arms, and Explosives (ATF) developed separate databases of images of

bullet and cartridge case markings, which could be queried to suggest pos-

sible matches. In 1996, the National Institute of Standards and Technology

(NIST) developed data exchange standards that permitted the integration

of the FBI’s DRUGFIRE database (cartridge case images) and the ATF’s

CEASEFIRE database (then limited to bullet images). The current National

Integrated Ballistic Information Network (NIBIN) includes images from

both cartridge cases and bullets that are associated with crime scenes and

is maintained by the ATF.

Periodically—and particularly in the wake of the Washington, DC,

⁵⁹ Although the metal and initial rifling are very similar, the cutting of the individual barrels,

the finishing machining, and the cleaning and polishing begin the process of differentiation of

the two sequentially manufactured barrels.

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sniper attacks in 2002—the question has been raised of expanding the scope

of databases like NIBIN to include images from test firings of newly manu-

factured firearms. In concept, this would permit downstream investigators

who recover a cartridge case or bullet at a crime scene to identify the likely

source firearm. Though two states (Maryland and New York) instituted

such reference ballistic image databases for newly manufactured firearms,

proposals to create such a database at the national level did not make sub-

stantial progress in Congress. A recent report of the National Academies,

Ballistic Imaging, examined this option in great detail and concluded that

“[a] national reference ballistic image database of all new and imported

guns is not advisable at this time.”⁶⁰

Analyses

In both firearm and toolmark identification, it is useful to distinguish

several types of characteristics that are considered by examiners. “Class

characteristics” are distinctive features that are shared by many items of the

same type. For example, the width of the head of a screwdriver or the pat-

tern of serrations in the blade of a knife may be class characteristics that are

common to all screwdrivers or knives of a particular manufacturer and/or

model. Similarly, the number of grooves cut into the barrel of a firearm and

the direction of “twist” in those grooves are class characteristics that can

filter and restrict the range of firearms that match evidence found at a crime

scene. “Individual characteristics” are the fine microscopic markings and

textures that are said to be unique to an individual tool or firearm. Between

these two extremes are “subclass characteristics” that may be common to

a small group of firearms and that are produced by the manufacturing pro-

cess, such as when a worn or dull tool is used to cut barrel rifling.

Bullets and cartridge cases are first examined to determine which class

characteristics are present. If these differ from a comparison bullet or car-

tridge, further examination may be unnecessary. The microscopic markings

on bullets and cartridge cases and on toolmarks are then examined under a

comparison microscope (made from two compound microscopes joined by

a comparison bridge that allows viewing of two objects at the same time).

The unknown and known bullet or cartridge case or toolmark surfaces

are compared visually by a firearms examiner, who can evaluate whether

a match exists.

⁶⁰ National Research Council. 2008. Ballistic Imaging. Washington, DC: The National

Academies Press, p. 5.

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Scientific Interpretation

The task of the firearms and toolmark examiner is to identify the indi-

vidual characteristics of microscopic toolmarks apart from class and sub-

class characteristics and then to assess the extent of agreement in individual

characteristics in the two sets of toolmarks to permit the identification of

an individual tool or firearm.

Guidance from the Association of Firearm and Tool Mark Examiners

(AFTE)⁶¹ indicates that an examiner may offer an opinion that a specific

tool or firearm was the source of a specific set of toolmarks or a particular

bullet striation pattern when “sufficient agreement” exists in the pattern

of two sets of marks. The standards then define agreement as significant

“when it exceeds the best agreement demonstrated between tool marks

known to have been produced by different tools and is consistent with the

agreement demonstrated by tool marks known to have been produced by

the same tool.”⁶²

Knowing the extent of agreement in marks made by different tools, and

the extent of variation in marks made by the same tool, is a challenging

task. AFTE standards acknowledge that these decisions involve subjective

qualitative judgments by examiners and that the accuracy of examiners’

assessments is highly dependent on their skill and training. In earlier years,

toolmark examiners relied on their past casework to provide a foundation

for distinguishing between individual, class, and subclass characteristics.

More recently, extensive training programs using known samples have

expanded the knowledge base of examiners.

The emergence of ballistic imaging technology and databases such as

NIBIN assist examiners in finding possible candidate matches between

pieces of evidence, including crime scene exhibits held in other geographic

locations. However, it is important to note that the final determination of

a match is always done through direct physical comparison of the evidence

by a firearms examiner, not the computer analysis of images. The growth

of these databases also permits examiners to become more familiar with

similarities in striation patterns made by different firearms. Newer imag-

ing techniques assess toolmarks using three-dimensional surface measure-

ment data, taking into account the depth of the marks. But even with

more training and experience using newer techniques, the decision of the

toolmark examiner remains a subjective decision based on unarticulated

⁶¹ Theory of identification, range of striae comparison reports and modified glossary

definitions—An AFTE Criteria for Identification Committee report. 1992. Journal of the As-

sociation of Firearm and Tool Mark Examiners 24:336-340.

⁶² Ibid., p. 336.

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standards and no statistical foundation for estimation of error rates.⁶³

The National Academies report, Ballistic Imaging, while not claiming to

be a definitive study on firearms identification, observed that, “The valid-

ity of the fundamental assumptions of uniqueness and reproducibility of

firearms-related toolmarks has not yet been fully demonstrated.” That

study recognized the logic involved in trying to compare firearms-related

toolmarks by noting that, “Although they are subject to numerous sources

of variability, firearms-related toolmarks are not completely random and

volatile; one can find similar marks on bullets and cartridge cases from the

same gun,” but it cautioned that, “A significant amount of research would

be needed to scientifically determine the degree to which firearms-related

toolmarks are unique or even to quantitatively characterize the probability

of uniqueness.”⁶⁴

Summary Assessment

Toolmark and firearms analysis suffers from the same limitations dis-

cussed above for impression evidence. Because not enough is known about

the variabilities among individual tools and guns, we are not able to specify

how many points of similarity are necessary for a given level of confidence

in the result. Sufficient studies have not been done to understand the reli-

ability and repeatability of the methods. The committee agrees that class

characteristics are helpful in narrowing the pool of tools that may have

left a distinctive mark. Individual patterns from manufacture or from wear

might, in some cases, be distinctive enough to suggest one particular source,

but additional studies should be performed to make the process of individu-

alization more precise and repeatable.

⁶³ Recent research has attempted to develop a statistical foundation for assessing the likeli-

hood that more than one tool could have made specific marks by assessing consecutive match-

ing striae, but this approach is used in a minority of cases. See A.A. Biasotti. 1959. A statistical

study of the individual characteristics of fired bullets. Journal of Forensic Sciences 4:34; A.A.

Biasotti and J. Murdock. 1984. “Criteria for identification” or “state of the art” of firearms

and tool marks identification. Journal of the Association of Firearms and Tool Mark Exam-

iners 16(4):16; J. Miller and M.M. McLean. 1998. Criteria for identification of tool marks.

Journal of the Association of Firearms and Tool Mark Examiners 30(1):15; J.J. Masson. 1997.

Confidence level variations in firearms identification through computerized technology. Journal

of the Association of Firearms and Tool Mark Examiners 29(1):42. For a critique of this area

and a comparison of scientific issues involving toolmark evidence and DNA evidence, see A.

Schwartz. 2004-2005. A systemic challenge to the reliability and admissibility of firearms and

tool marks identification. Columbia Science and Technology Law Review 6:2. For a rebuttal

to this critique, see R.G. Nichols. 2007. Defending the scientific foundations of the firearms

and tool mark identification discipline: Responding to recent challenges. Journal of Forensic

Sciences 52(3):586-594.

⁶⁴ All quotes from National Research Council. 2008. Ballistic Imaging. Washington, DC:

The National Academies Press, p. 3.

FORENSIC SCIENCE DISCIPLINES

155

A fundamental problem with toolmark and firearms analysis is the

lack of a precisely defined process. As noted above, AFTE has adopted a

theory of identification, but it does not provide a specific protocol. It says

that an examiner may offer an opinion that a specific tool or firearm was

the source of a specific set of toolmarks or a bullet striation pattern when

“sufficient agreement” exists in the pattern of two sets of marks. It defines

agreement as significant “when it exceeds the best agreement demonstrated

between tool marks known to have been produced by different tools and

is consistent with the agreement demonstrated by tool marks known to

have been produced by the same tool.” The meaning of “exceeds the best

agreement” and “consistent with” are not specified, and the examiner is

expected to draw on his or her own experience. This AFTE document,

which is the best guidance available for the field of toolmark identification,

does not even consider, let alone address, questions regarding variability,

reliability, repeatability, or the number of correlations needed to achieve a

given degree of confidence.

Although some studies have been performed on the degree of similarity

that can be found between marks made by different tools and the vari-

ability in marks made by an individual tool, the scientific knowledge base

for toolmark and firearms analysis is fairly limited. For example, a report

from Hamby, Brundage, and Thorpe⁶⁵ includes capsule summaries of 68

toolmark and firearms studies. But the capsule summaries suggest a heavy

reliance on the subjective findings of examiners rather than on the rigorous

quantification and analysis of sources of variability. Overall, the process for

toolmark and firearms comparisons lacks the specificity of the protocols

for, say, 13 STR DNA analysis. This is not to say that toolmark analysis

needs to be as objective as DNA analysis in order to provide value. And,

as was the case for friction ridge analysis and in contrast to the case for

DNA analysis, the specific features to be examined and compared between

toolmarks cannot be stipulated a priori. But the protocols for DNA analysis

do represent a precisely specified, and scientifically justified, series of steps

that lead to results with well-characterized confidence limits, and that is the

goal for all the methods of forensic science.

ANALYSIS OF HAIR EVIDENCE

The basis for hair analyses as forensic evidence stems from the fact that

human and animal hairs routinely are shed and thus are capable of being

⁶⁵ J.E. Hamby, D.J. Brundage, and J.W. Thorpe. 2009. The identification of bullets fired

from 10 consecutively rifled 9mm Ruger pistol barrels—A research project involving 468

participants from 19 countries. Available online at http://www.fti-ibis.com/DOWNLOADS/

Publications/10%20Barrel%20Article-%20a.pdf.

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transferred from an individual to the crime scene, and from the crime scene

to an individual. Forensic hair examiners generally recognize that various

physical characteristics of hairs can be identified and are sufficiently differ-

ent among individuals that they can be useful in including, or excluding,

certain persons from the pool of possible sources of the hair. The results of

analyses from hair comparisons typically are accepted as class associations;

that is, a conclusion of a “match” means only that the hair could have come

from any person whose hair exhibited—within some levels of measurement

uncertainties—the same microscopic characteristics, but it cannot uniquely

identify one person. However, this information might be sufficiently useful

to “narrow the pool” by excluding certain persons as sources of the hair.

Although animal hairs might provide useful evidence in certain cases

(e.g., animal poaching), animal hair analysis often can lead to an identifica-

tion of only the type of animal, not the specific breed⁶⁶; consequently, most

(90 to 95 percent) of hair analyses refer to analyses of human hair. Human

hairs from different parts of the body have different characteristics; Houck

cautions strongly against drawing conclusions about hairs from one part of

the body based on analyses of hairs from a different body part.⁶⁷

Houck and Bisbing recommend as minimal training for hair examiners

a bachelor’s degree in a natural or applied science (e.g., chemistry, biology,

forensic science), on-the-job training programs, and an annual proficiency

test.⁶⁸

Sample Data and Collection

Sample hairs received for analysis initially are examined macroscopi-

cally for certain broad features such as color, shaft form (e.g., straight,

wavy, curved, kinked), length, and overall shaft thickness (e.g., fine, me-

dium, coarse).

In the second stage of analysis, hairs are mounted on microscopic slides

using a mounting medium that has the same refractive index (about 1.54)

as the hair, to better view the microscopic features (see next section). One

hair or multiple hairs from the same source may be mounted on a glass

microscope slide with an appropriate cover slip, as long as each mounted

hair is clearly visible. It is most important that questioned and known hairs

are mounted in the same type of mounting medium.

During this examination, the hair analyst attempts to identify the part

of the body from which the hair might have come, based on certain de-

⁶⁶ P.D. Barnett and R.R. Ogle. 1982. Probabilities and human hair comparison. Journal of

Forensic Sciences 27(2):272-278.

⁶⁷ M.M. Houck and R.E. Bisbing. 2005. Forensic human hair examination and comparison

in the 21st century. Forensic Science Review 17(1):7.

⁶⁸ Ibid., p. 12.

FORENSIC SCIENCE DISCIPLINES

157

finable characteristics that distinguish hairs from various body locations.

Occasionally, suspects can be eliminated on the basis of these simple mi-

croscopic characteristics.

A “control” or “comparison” group of hairs must be collected from a

known hair source. A known head hair sample should consist of hairs from

the five different areas of the scalp (top, front, back including nape, and

both sides). Known hair samples should be obtained by a combination of

pulling and combing from the sampled region. Ideally, a total of 50 hairs

should be obtained from the scalp. A known pubic hair sample or a sample

from any other somatic region should ideally consist of 25 hairs obtained

by pulling and combing from different regions. A comparison can still be

performed with less than the recommended number of hairs, but this may

increase the likelihood of a false exclusion.⁶⁹

Features from human hair analyses can be divided broadly into “ma-

jor characteristics” and “secondary characteristics.” The former category

includes features such as color, treatment (e.g., dyed, bleached, curled,

permed), pigment aggregation (e.g., streaked, clumped, patchy), and shaft

form (e.g., wavy, straight, curly). Other major characteristics may include

pigment distribution (e.g., uniform, peripheral, clustered), medulla appear-

ance, if present (e.g., continuous, interrupted, or fragmented—and opaque

or translucent), hair diameter, medullary index, and presence or absence of

cortical fusi (e.g., root or shaft). Secondary characteristics include cuticular

margin (e.g., smooth, serrated, looped, or cracked), pigment density (e.g.,

absent, sparse, heavy), pigment size (e.g., absent, fine, coarse), tip shape

(e.g., tapered, cut, rounded, frayed, split), and shaft diameter (e.g., narrow

or wide).⁷⁰

Studies of Accuracy in Identification

In 1974, investigators Gaudette and Keeping described a system of hair

analysis and used it in a study of pairwise comparisons among 861 hairs

from 100 different persons.⁷¹ They acknowledged that “the hair samples

were not chosen from the population at random, but were selected so that

the probability of two hairs being similar would be greater, if anything,

than in the population at large.”⁷² From their assignment of probabilities,

the authors estimated that the chance of asserting a difference between two

⁶⁹ Scientific Working Group on Materials Analysis (SWGMAT). 2005. Forensic human

hair examination guidelines. Forensic Science Communications 7(2). Available at www.fbi.

gov/hq/lab/fsc/backissu/april2005/standards/2005_04_standards02.htm.

⁷⁰ Ibid.

⁷¹ B.D. Gaudette and E.S. Keeping. 1974. An attempt at determining probabilities in human

scalp hair comparison. Journal of Forensic Sciences 19(3):599-606.

⁷² Ibid., p. 65.

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hairs from the same person is small, about 1 in 4,500.⁷³ This assignment of

probabilities has since been shown to be unreliable.⁷⁴ Moreover, the study

does not confirm the chance of asserting a match between two dissimilar

hairs, and the authors acknowledge that, “due to the fact that so many of

the characteristics coded are subjective—for example, color, texture—it was

not possible to get complete reproducibility between two or more examin-

ers coding the same hair.”⁷⁵

Barnett and Ogle raised four concerns with the Gaudette and Keeping

study: (1) it relied on idealized (not from real life) test scenarios; (2) there

was no objective basis for selecting the features; (3) the statistical analysis

of data from the study was questionable; and (4) there was a possible exam-

iner bias.⁷⁶ Gaudette attempted to address these concerns through a further

study. However, this additional study involved only three hair examiners,

in addition to the author. The author concluded that:

. . . whereas hair is not generally a basis for positive personal identification, the

presence of abnormalities or unusual features or the presence of a large number of

different unknown hairs all similar to the standard can lead to a more positive con-

clusion. The problem, at present, lies in finding suitable additional characteristics

[of hair, for effecting individualization]. Although there is basic agreement as to the

value of the macroscopic and microscopic characteristics used, other characteristics

are either unreliable or controversial. Physical characteristics such as refractive

index, density, scale counts, tensile strength, and electrical properties have been

proposed by some workers but have been attacked by others, and the general con-

sensus is that they are of little use in hair comparison.⁷⁷

In 1990, Wickenheiser and Hepworth attempted a study to address

examiner bias in a small study with only two examiners. They reported that

“no incorrect associations were made by either examiner.”⁷⁸ But a study

with only two examiners cannot offer accurate and precise estimates of bias

in the population of examiners.

An attempt at an objective system for identifying “matches” among

hair samples is presented in Verma et al., based on a neural network.⁷⁹

⁷³ A later study on human pubic hairs (Caucasian only) estimated this probability as “about

1 in 800.” B.D. Gaudette. 1976. Probabilities and human pubic hair comparisons. Journal of

Forensic Sciences 21(3):514-517.

⁷⁴ P.D. Barnett and R.R. Ogle. 1982. Probabilities and human hair comparison. Journal of

Forensic Sciences 27(2):272-278.

⁷⁵ Gaudette and Keeping, op. cit.

⁷⁶ Barnett and Ogle, op. cit.

⁷⁷ B.D. Gaudette. 1978. Some further thoughts on probabilities and human hair compari-

sons. Journal of Forensic Sciences 23(4):758-763, pp. 761-762.

⁷⁸ Wickenheiser and Hepworth, op. cit., p. 1327.

⁷⁹ M.S. Verma, L. Pratt, C. Ganesh, and C. Medina. 2002. Hair-MAP: A prototype au-

tomated system for forensic hair comparison and analysis. Forensic Science International

129:168-186.

FORENSIC SCIENCE DISCIPLINES

159

According to the authors of this article, “The system accurately judged

whether two populations of hairs came from the same person or from dif-

ferent persons 83 percent of the time.”⁸⁰ The article states that 83 percent

was obtained by testing the neural network on all possible pairs among 9

samples of hairs from 9 people (i.e., 81 combinations, of which 9 are “true

matches” and 72 are “true mismatches”). Their Table 3⁸¹ can be summa-

rized as follows:

System said

“same”

“different”

Same person

Total= 9

Different persons

Total=73

Because the total of these 4 numbers is 82, not 81, one presumes a

typographical error in the table; as stated, the number of correct calls is

(5 + 64)/81=0.85, or 85 percent. (If one of the counts, 5 or 64, is off by 1,

the percentage would be 84 percent.) However, the table also shows that

the neural network claimed 9 of the 73 different pairs as “same,” for a false

positive rate of 9/73=12 percent, and 4 sets of hairs from the same person

as “different,” for a false negative rate of 4/9=44 percent. With such high

error rates, one would want to study improvements to such systems before

putting them into routine practice.

Houck et al. indicate that proficiency testing is conducted regularly

for hair experts in crime laboratories.⁸² Collaborative Testing Services⁸³

offers hair and fiber proficiency tests annually. Unfortunately, mass produc-

tion of test samples such as hair is problematic. Because known samples

exhibit a range of characteristics within each of the major and secondary

characteristics, it is not possible to provide comparable samples to multiple

examiners.

Scientific Interpretation and Reporting of Results

The success of hair analyses to make a positive identification is lim-

ited in important ways. Most hair examiners would opine only that hairs

exhibiting the same microscopic characteristics “could” have come from a

⁸⁰ Ibid., p. 179.

⁸¹ Ibid., p. 180.

⁸² M.M. Houck, R.E. Bisbing, T.G. Watkins, and R.P. Harman. 2004. Locard exchange: The

science of forensic hair comparisons and the admissibility of hair comparison evidence: Frye

and Daubert considered. Modern Microscopy Journal Available at www.modernmicroscopy.

com/main.asp?article=36&searchkeys=Houck%2BBisbing.

⁸³ See www.collaborativetesting.com.

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particular individual. Moreover, the “best” or most reliable characteristics

will vary by case. For example, “color” may be a critical determinant in a

case where it is artificial, because that introduces additional independent

variables, such as the time since treatment and the actual hair color, while

a natural hair might provide less information.

However, several members of the committee have experienced court-

room cases in which, despite the lack of a statistical foundation, microscopic

hair examiners have made probabilistic claims based on their experience,

as occurred in some DNA exoneration cases in which microscopic hair

analysis evidence had been introduced during trial. Aitken and Robertson

discuss some probabilistic concepts with respect to hair analysis.⁸⁴

The availability of DNA analysis has lessened the reliance on hair

examination. In a very high proportion of cases involving hair evidence,

DNA can be extracted, even years after the crime has been committed.

Although the DNA extraction may consist of only mitochondrial DNA

(mtDNA), such analyses are likely to be much more specific than those

conducted on the physical features of hair. For this reason, cases that might

have relied heavily on hair examinations have been subjected more recently

to additional analyses using DNA.⁸⁵ Because of the inherent limitations of

hair comparisons and the availability of higher-quality and higher-accuracy

analyses based on mtDNA, traditional hair examinations may be presented

less often as evidence in the future, although microscopic comparison of

physical features will continue to be useful for determining which hairs are

sufficiently similar to merit comparisons with DNA analysis and for exclud-

ing suspects and assisting in criminal investigations.

Summary Assessment

No scientifically accepted statistics exist about the frequency with

which particular characteristics of hair are distributed in the population.

There appear to be no uniform standards on the number of features on

which hairs must agree before an examiner may declare a “match.” In one

study of validity and accuracy of the technique, the authors required exact

agreement on seven “major” characteristics and at least two agreements

among six “secondary” characteristics.⁸⁶ The categorization of hair features

depends heavily on examiner proficiency and practical experience.

An FBI study found that, of 80 hair comparisons that were “associ-

⁸⁴ C.G.G. Aitken and J.A. Robertson. 1986. A contribution to the discussion of probabilities

and human hair comparisons. Journal of Forensic Sciences 32(3):684-689.

⁸⁵ M.M. Houck and B. Budowle. 2002. Correlation of microscopic and mitochondrial DNA

hair comparisons. Journal of Forensic Sciences 47(5):964-967.

⁸⁶ R.A. Wickenheiser and D.G. Hepworth. 1990. Further evaluation of probabilities in hu-

man hair comparisons. Journal of Forensic Sciences 35(6):1323-1329.

FORENSIC SCIENCE DISCIPLINES

161

ated” through microscopic examinations, 9 of them (12.5 percent) were

found in fact to come from different sources when reexamined through

mtDNA analysis.⁸⁷ This illustrates not only the imprecision of microscopic

hair analyses, but also the problem with using imprecise reporting terminol-

ogy such as “associated with,” which is not clearly defined and which can

be misunderstood to imply individualization.

In some recent cases, courts have explicitly stated that microscopic hair

analysis is a technique generally accepted in the scientific community.⁸⁸ But

courts also have recognized that testimony linking microscopic hair analysis

with particular defendants is highly unreliable.⁸⁹ In cases where there seems

to be a morphological match (based on microscopic examination), it must

be confirmed using mtDNA analysis; microscopic studies alone are of lim-

ited probative value. The committee found no scientific support for the use

of hair comparisons for individualization in the absence of nuclear DNA.

Microscopy and mtDNA analysis can be used in tandem and may add to

one another’s value for classifying a common source, but no studies have

been performed specifically to quantify the reliability of their joint use.

ANALYSIS OF FIBER EVIDENCE

Fibers associated with a crime—including synthetic fibers such as nylon,

polyester and acrylic as well as botanical fibers such as ramie or jute, which

are common in ropes or twines—can be examined microscopically in the

same way as hairs, and with the same limitations. However, fibers also can

be analyzed using the tools of analytical chemistry, which provide a more

solid scientific footing than that underlying morphological examination. In

some cases, clothing and carpets have been subjected to relatively distinctive

environmental conditions (e.g., sunlight exposure or laundering agents) that

impart characteristics that can distinguish particular items from others from

the same manufacturing lot. Fiber examiners agree, however, that none of

these characteristics is suitable for individualizing fibers (associating a fiber

from a crime scene with one, and only one, source) and that fiber evidence

can be used only to associate a given fiber with a class of fibers.⁹⁰

⁸⁷ Houck and Budowle, op. cit.

⁸⁸ E.g., State v. West, 877 A.2d 787 (Conn. 2005); Bookins v. State, 922 A.2d 389 (Del.

Supr, 2007).

⁸⁹ See P.C. Giannelli and E. West. 2001. Hair comparison evidence. Criminal Law Bulletin

37:514.

⁹⁰ See, e.g., R.R. Bresee. 1987. Evaluation of textile fiber evidence: A review. Journal of

Forensic Sciences 32(2):510-521. See also SWGMAT. 1999. Introduction to forensic fiber

examination. Forensic Science Communications 1(1). Available at www.fbi.gov/hq/lab/fsc/

backissu/april1999/houcktoc.htm, which includes the following summarization in Section 5.4:

“It can never be stated with certainty that a fiber originated from a particular textile because

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Another type of fiber analysis consists of physically matching two rem-

nants that appear to be torn from one another. By comparing the shapes of

the mating edges, and aligning any patterns in the cloth, it can sometimes be

possible to associate a fragment with the garment or other item from which

it was torn. This is a form of pattern matching, analogous to the matching

of shoe and tire prints, but it will not be discussed further here.

Sample Collection and Analysis

The collection of fibers and of a comparison group follows the same

procedures as those for mounting hairs. If a macroscopic analysis (e.g., or

color, texture, shape) suggests that the two samples appear to be the same,

additional procedures such as the following are pursued:

1. Microscopy (reflected light)

2. Polarized light microscopy/fluorescence microscopy

3. Infrared microscopy (to determine man-made fiber composition,

such as nylon, polyester)

4. Solubility in a medium

5. Melting point

6. Cross-sectional shape

7. Pyrolysis GC

8. Microspectrophotometry (MSP)

9. Raman spectroscopy

The last of these, Raman spectroscopy, often can provide additional

information on polymer chain length (short, medium, long) and branching.

Its use in forensic laboratories is rare, although research is under way to

develop possible applications. A good overview of fiber evidence is provided

by Grieve and Robertson.⁹¹

Summary Assessment

A group of experienced paint examiners, the Fiber Subgroup of the

Scientific Working Group on Materials Analysis (SWGMAT), has produced

guidelines,⁹² but no set standards, for the number and quality of character-

other textiles are produced using the same fiber types and color. The inability to positively

associate a fiber to a particular textile to the exclusion of all others, however, does not mean

that a fiber association is without value.”

⁹¹ M. Grieve and J. Robertson. 1999. Forensic Examination of Fibres. London: Taylor and

Francis Ltd.

⁹² SWGMAT, op. cit. Available at www.fbi.gov/hq/lab/fsc/backissu/april1999/houcktoc.

htm.

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163

istics that must correspond in order to conclude that two fibers came from

the same manufacturing batch. There have been no studies of fibers (e.g.,

the variability of their characteristics during and after manufacturing) on

which to base such a threshold. Similarly, there have been no studies to

inform judgments about whether environmentally related changes discerned

in particular fibers are distinctive enough to reliably individualize their

source, and there have been no studies that characterize either reliability or

error rates in the procedures. Thus, a “match” means only that the fibers

could have come from the same type of garment, carpet, or furniture; it can

provide only class evidence.

Because the analysis of fibers is made largely through well-character-

ized methods of chemistry, it would be possible in principle to develop an

understanding of the uncertainties associated with those analyses.⁹³ How-

ever, to date, that has not been done. Fiber analyses are reproducible across

laboratories because there are standardized procedures for such analyses.

Proficiency tests are routinely provided and taken annually, and the reports

are available from Collaborative Testing Services.

QUESTIONED DOCUMENT EXAMINATION⁹⁴

Questioned document examination involves the comparison and analy-

sis of documents and printing and writing instruments in order to identify

or eliminate persons as the source of the handwriting; to reveal alterations,

additions, or deletions; or to identify or eliminate the source of typewriting

or other impression marks. Questions about documents arise in business,

finance, and civil and criminal trials, and in any matter affected by the in-

tegrity of written communications and records. Typical analyses include:

• determining whether the document is the output of mechanical or

electronic imaging devices such as printers, copying machines, and

facsimile equipment;

• identifying or eliminating particular human or machine sources of

handwriting, printing, or typewriting;

• identifying or eliminating ink, paper, and writing instrument;

• establishing the source, history, sequence of preparation, altera-

tions or additions to documents, and relationships of documents;

⁹³ Some relevant questions to be addressed are identified in Bresee, op. cit.

⁹⁴ This discussion is primarily based on Standard Descriptions of Scope of Work Relating to

Forensic Document Examiners (American Society for Testing and Materials [ASTM] Designa-

tion E 444-98) (1998), Standard Guide for Test Methods for Forensic Writing Ink Comparison

(ASTM Designation E 1422-01) (2001), Standard Guide for Writing Ink Identification (ASTM

Designation E 1789-04) (2004), and Standard Guide for Examination of Handwritten Items

(ASTM Designation E 2290-03) (2003).

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• deciphering and restoring obscured, deleted, or damaged parts of

documents;

• recognizing and preserving other physical evidence that may be

present in documents; and

• determining the age of a document.

Questioned document examiners are also referred to as forensic docu-

ment examiners or handwriting experts; questioned document examination

includes the field of handwriting identification, while handwriting includes

cursive or script style writing, printing by hand, signatures, numerals, or

other written marks or signs. Forensic document examination does not

involve a study of handwriting that purports to create a personality profile

or otherwise analyze or judge the writer’s personality or character.

Analyses

Equipment used in questioned document examination includes micro-

scopes and other optical aids, photographic and other imaging devices,

and a wide variety of imaging materials adaptable for use with numerous

lighting methods, including those involving ultraviolet, visible, and infrared

light, and other regions of the electromagnetic spectrum. Software tools re-

cently have become available for the analysis of handwriting.⁹⁵ The analysis

of papers and inks is similar to other forensic chemistry work. The principal

procedures used for ink examination are nondestructive optical examina-

tions and chemical examinations. Optical examinations include those that

use visible and alternative light sources—for example, determining whether

the class of ink is ballpoint pen; using ultraviolet examination to reveal

indications that a document has been stained by chemicals; and employing

reflected infrared to observe luminescence at different wavelengths. Chemi-

cal examination includes spot testing during which solvents are applied in

small amounts to the ink line. For example, ballpoint inks, which are either

oil based or glycol based, are highly soluble in pyridine. Inks formulated

for fountain pens, porous point pens, and roller pens generally are water

soluble in ethanol and water. Indelible markers are solvent based and gener-

ally would be soluble in pyridine.

Ink examination can have one of two objectives: class identification—

for which the intention is to identify the ink formula or type based on a

reference library of samples of inks—and comparison, for which the goal

is to compare two ink samples to determine whether they are of common

⁹⁵ For an overview, see S.N. Srihari and G. Leedham. 2003. A survey of computer methods in

forensic document examination. Proceedings of the 11th International Graphonomics Society

Conference, pp. 278-281. Available at www.ntu.edu.sg/sce/labs/forse/PDF/docExam_7.pdf.

FORENSIC SCIENCE DISCIPLINES

165

origin. Ink comparisons usually are performed to answer four basic cat-

egories of questions: (1) whether an ink is the same (in formula) as that

on other parts of the same document or on other documents; (2) whether

two writings with similar ink have a common origin (e.g., the same writing

instrument or ink well); (3) whether the ink of entries over a period of time

is consistent with varying ages or indicates preparation at one time; and (4)

whether ink is as old as it purports to be.

Most problems with ink examinations arise from confounding factors

that interact with the ink. These can be part of the writing process, such as

blotting wet ink; variations in the papers; various forms of contamination

on the document; or a combination of these factors. Most ink examinations

must be performed on paper and without defacing the handwriting, and this

creates a number of sampling and analytical challenges.

The examination of handwritten items typically involves the compari-

son of a questioned item submitted for examination along with a known

item of established origin associated with the matter under investigation.

Requirements for comparison are that the writing be of the same type

(handwritten/cursive versus hand printed) and that it be comparable text

(similar letter/word combinations). Special situations involving unnatural

writing are forgery (an attempt to imitate/duplicate the writing of another

person) and disguise (an attempt to avoid identification as the writer). The

basis for comparison is that handwriting/handprinting/numerals can be

examined to obtain writing characteristics (also referred to as features or

attributes). The characteristics are further classified into class characteristics

(the style that the writer was taught), individual characteristics (the writer’s

personal style), and gross/subtle characteristics.

Specific attributes used for comparison of handwriting are also referred

to as discriminating elements, of which Huber and Headrick have identified

21.⁹⁶ Comparisons are based on the high likelihood that no two persons

write the same way, while considering the fact that every person’s writing

has its own variabilities. Thus, an analysis of handwriting must compare

interpersonal variability—some characterization of how handwriting fea-

tures vary across a population of possible writers—with intrapersonal

variability—how much an individual’s handwriting can vary from sample

to sample. Determining that two samples were written by the same person

depends on showing that their degree of variability, by some measure,

is more consistent with intrapersonal variability than with interpersonal

variability. Some cases of forgery are characterized by signatures with too

little variability, and are thus inconsistent with the fact that we all have

intrapersonal variability in our writing.

⁹⁶ R.A. Huber andA. M. Headrick. 1999. Handwriting Identification: Facts and Fundamen-

tals. Boca Raton, FL: CRC Press.

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STRENGTHENING FORENSIC SCIENCE IN THE UNITED STATES

Scientific Interpretation and Reporting of Results

Terminology has been developed for expressing the subjective conclu-

sions of handwriting comparison and identification, taking into account

that there are an infinite number of gradations or opinions toward an

identification or elimination. Several scales, such as a five-point scale and a

nine-point scale, are used by questioned document examiners worldwide.

The nine-point scale is as follows:

1. Identification (a definite conclusion that the questioned writing

matches another sample)

2. Strong probability (evidence is persuasive, yet some critical quality

is missing)

3. Probable (points strongly towards identification)

4. Indications [that the same person] did [create both samples] (there

are a few significant features)

5. No conclusion (used when there are limiting factors such as dis-

guise, or lack of comparable writing)

6. Indications [that the same person] did not [create both samples]

(same weight as indications with a weak opinion)

7. Probably did not (evidence is quite strong)

8. Strong probably did not (virtual certainty)

9. Elimination (highest degree of confidence)⁹⁷

Summary Assessment

The scientific basis for handwriting comparisons needs to be strength-

ened.⁹⁸ Recent studies have increased our understanding of the individuality

and consistency of handwriting and computer studies⁹⁹ and suggest that

⁹⁷ Standard Terminology for Expressing Conclusions of Forensic Document Examiners,

ASTM Designation E 1658-04.

⁹⁸ M. Kam, G. Fielding, and R. Conn. 1997. Writer identification by professional document

examiners. Journal of Forensic Sciences 42(5):778-786, reports on proficiency tests given to

more than 100 questioned document examiners and to a control group of individuals with

similar educational backgrounds. Each subject made 144 pair-wise comparisons. Although

the study showed that document examiners are much more accurate than lay people in de-

termining whether or not two samples “match” (based on the “identification” and “strong

probability” definitions of ASTM standard E1658), professionals nonetheless declared an

erroneous match in 6.5 percent of the comparisons. A similar, more recent study, focusing on

whether individual signatures were genuine, is reported in J. Sita, B. Found, and D. Rogers.

2002. Forensic handwriting examiners’ expertise for signature comparison. Journal of Foren-

sic Sciences 47:1117. That study found that professional handwriting examiners erred in 3.4

percent of their judgments.

⁹⁹ E.g., S.N. Sargur, S.-H. Cha, H. Arora, and S. Lee. 2002. Individuality of handwriting.

Journal of Forensic Sciences 47(4):1-17.

FORENSIC SCIENCE DISCIPLINES

167

there may be a scientific basis for handwriting comparison, at least in the

absence of intentional obfuscation or forgery. Although there has been only

limited research to quantify the reliability and replicability of the practices

used by trained document examiners, the committee agrees that there may

be some value in handwriting analysis.

Analysis of inks and paper, being based on well-understood chemistry,

presumably rests on a firmer scientific foundation. However, the committee

did not receive input on these fairly specialized methods and cannot offer a

definitive view regarding the soundness of these methods or of their execu-

tion in practice.

ANALYSIS OF PAINT AND COATINGS EVIDENCE

Paint is a suspension of solid pigments in a polymeric binder that, after

application by brushing, spraying, dipping, or other means, forms a protec-

tive and/or decorative coating. When two objects come in contact with one

another and at least one of these objects is painted, a transfer of paint may

occur. This transferred paint can be compared to the paint located near the

point of damage to determine if the two samples have a common origin.

Painted surfaces tend to be repainted over time, providing a characteristic

history of layer sequence. Painted surfaces are encountered frequently at

crime scenes in the form of vehicles, architectural structures, tools, bicycles,

boats, and many other items. The results of the examinations often are

valuable both during the investigation and as evidence if a trial results.

Paint examinations by their nature can be useful in suggesting possible con-

nections of evidence from the crime scene to its source and therefore are

helpful in narrowing or excluding possible witnesses and suspects as well

as in providing useful information for investigative leads.

Sample Data and Collection

There are many different types of paint and other coatings, including

architectural, vehicular, and marine. Evidence collected from the crime

scene may include painted surfaces such as automotive panels, tools, or

victims’ or suspects’ clothing, or spray paint, smears, chips, or flakes. After

documentation at the scene, the damaged painted surface is protected and

preserved and then submitted to the laboratory. When it is not possible to

bring the painted item or a portion of it to the laboratory, paint samples

may be removed in such a way that the entire layer sequence is captured

intact.

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STRENGTHENING FORENSIC SCIENCE IN THE UNITED STATES

Analyses

The proper recognition and collection of paint evidence at the scene

precedes the comparison of evidence occurring at the laboratory. The color,

texture, type, layer sequence, and chemical composition of known and

questioned paints are compared, and a conclusion is rendered. Additionally,

in cases for which no suspect vehicle and questioned paint are available, it

may be possible to provide at least an investigative lead based on the color

and metallic/nonmetallic type of paint present. If appropriate, the Royal

Canadian Mounted Police’s PDQ (Paint Data Query) database may be

searched, and vehicular information may be provided regarding the pos-

sible makes, models, and year range of vehicles that used the questioned

paint system.

The examination and comparison of paint evidence requires microscopic

and instrumental techniques and methods. The examination of questioned

and known samples follows an analytical process that identifies and com-

pares the class (or group) characteristics of the evidence.¹⁰⁰ Occasionally,

identifying characteristics exist across edges that allow edge or piece fitting.

These characteristics include irregular borders, brush stroke striations, pol-

ish mark striations, or surface abrasion markings. When paint fragments

physically fit back to a sample from a known source, the fragments are

identified as having come from that specific source. Only when physical

fitting is possible can an individualized source determination be made

Examiners involved with the analysis of paint evidence in the labora-

tory typically possess an extensive scientific background, because many

of the methods and analyses rely heavily on chemistry.¹⁰¹ The suggested

minimum education requirement is a bachelor’s degree in a natural^102,103

or applied science,¹⁰⁴ with many candidates possessing a graduate degree.

Coursework needs to include one year (or equivalent) of general chemistry

with laboratory, organic chemistry with laboratory, analytical/instrumental

analysis, and light microscopy to include basic polarized light microscopy—

the latter obtained through structured coursework if it is not available at

the graduate or undergraduate level.¹⁰⁵ On-the-job training continues in the

laboratory, with its length depending on the examiner’s experience. Before

examiner trainees can work cases independently, they must observe and

¹⁰⁰ SWGMAT. 1999. Forensic paint analysis and comparison guidelines. Forensic Science

Communications 1(2). Available at www.fbi.gov/hq/lab/fsc/backissu/july1999/painta.htm.

¹⁰¹ SWGMAT. 2000. Trace evidence quality assurance guidelines. Forensic Science Com-

munications 2(1). Available at www.fbi.gov/hq/lab/fsc/backissu/jan2000/swgmat.htm.

¹⁰² G.S. Anderson (ed.). Canadian Society of Forensic Science. 2007. CSFS Careers in Foren-

sic Science, p. 15. Available at www.csfs.ca/contentadmin/UserFiles/File/Booklet2007.pdf.

¹⁰³ SWGMAT 2000, op. cit.

¹⁰⁴ Ibid.

¹⁰⁵ Ibid.

FORENSIC SCIENCE DISCIPLINES

169

work under the supervision of an experienced examiner. The completion

of a laboratory’s training program in paint analysis can range between 12

to 18 months.¹⁰⁶

Scientific Interpretation and Reporting of Results

SWGMAT sets guidelines for this field, but it has not recommended

report wording, and there are no set criteria for determining a conclusion,

although a range of conclusions may be used to show the significance of the

examination results. The strength of a conclusion depends on such variables

as the number of layers present, the sample condition, and the type of paint

(vehicular or structural). Terms such as “matched,” “indistinguishable,”

“consistent,” or “similar” are used along with the properties of the paints

that were compared in stating the results of the comparison.

If there are no significant differences in the properties compared, the

examiners may conclude that the paint or coating samples could have had

a common origin. This does not mean they came from the same source to

the exclusion of all others, but rather that they may have originated from

the same source or from different sources that were painted or coated in the

same manner. As the number of different layers associated increases (e.g.,

multiple different layers on a repainted surface), it may be concluded that it

is unlikely that the questioned paint originated from any source other than

that of the known paint.

SWGMAT has suggested forensic paint analysis and comparison guide-

lines^107,108 that discuss the examination procedure and instrumentation op-

tions, and ASTM has published the general guidelines.¹⁰⁹ However, neither

includes report wording suggestions. Additional work should be done to

provide standard language for reporting conclusions and sources of uncer-

tainty. Such work has been completed by working groups for other forensic

disciplines. Proficiency testing requirements are agreed upon by the predom-

inant accrediting organization, the American Society of Crime Laboratory

Directors-Laboratory Accreditation Board (ASCLD/LAB), which requires

testing (internal or external) once per calendar year.

¹⁰⁶ Anderson, op. cit.; SWGMAT.

¹⁰⁷ SWGMAT. 1999. Forensic paint analysis and comparison guidelines. Forensic Science

Communications 1(2). Available at www.fbi.gov/hq/lab/fsc/backissu/july1999/painta.htm.

¹⁰⁸ SWGMAT. 2002. Standard guide for using scanning electron microscopy/X-ray spec-

trometry in forensic paint examinations. Forensic Science Communications 4(4). Available at

www.fbi.gov/hq/lab/fsc/backissu/oct2002/bottrell.htm.

¹⁰⁹ Ibid.

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STRENGTHENING FORENSIC SCIENCE IN THE UNITED STATES

Summary Assessment

As is the case with fiber evidence, analysis of paints and coatings is

based on a solid foundation of chemistry to enable class identification. Vi-

sual and microscopic examinations are typically the first step in a forensic

examination of paints and coatings because of the ability to discriminate

paints/coatings based on properties determined with these examinations.

Several studies have been conducted that included hundreds of random

automotive paint samples.¹¹⁰ These studies have concluded that more than

97 percent of the samples could be differentiated based on microscopic

examinations coupled with solubility and microchemical testing. Another

study¹¹¹ determined that more than 99 percent of 2,000 architectural paint

samples could be similarly differentiated. However, the community has not

defined precise criteria for determining whether two samples come from a

common source class.

ANALYSIS OF EXPLOSIVES EVIDENCE AND FIRE DEBRIS

Explosives evidence encompasses a wide range of materials from un-

burned, unconsumed powders, liquids, and slurries, to fragments of an ex-

plosive device, to objects in the immediate vicinity of an explosion thought

to contain residue from the explosive. A typical analytical approach would

be to identify the components and construction of an explosive device and

conduct an analysis of any unconsumed explosives and residues. In addition

to the analysis and identification of low and high explosives, chemical reac-

tion bottle bombs are also analyzed. The scene of an explosion can require

special investigative attention. What may appear to be a small piece of scrap

metal could in fact be an important piece of the device that caused the ex-

plosion. The very nature of an explosion has a direct impact on the quality

of evidence recovered. Pristine devices or device fragments, or appreciable

amounts of unconsumed explosive material, should not be expected.

Analyses

Generally speaking, laboratories will not accept devices until they have

been rendered safe. Examiners involved with the analysis of explosives evi-

dence in the laboratory typically have an extensive scientific background,

because the methods used entail a large amount of chemistry and instru-

¹¹⁰ S.G. Ryland and R.J. Kopec. 1979. The evidential value of automobile paint chips. Jour-

nal of Forensic Sciences 24(1):140-147; J.A. Gothard. 1976. Evaluation of automobile paint

flakes as evidence. Journal of Forensic Sciences 21(3):636-641.

¹¹¹ C.F. Tippet. 1968. The evidential value of the comparison of paint flakes from sources

other than vehicles. Journal of the Forensic Sciences Society 8(2-3):61-65.

FORENSIC SCIENCE DISCIPLINES

171

mentation. The Technical Working Group for Fire and Explosives (TW-

GFEX), a group of fire debris and explosives examiners, suggests that an

explosives examiner be required to possess a bachelor’s degree in a natural

or applied science, with recommended coursework in chemistry and instru-

mental analysis.¹¹² The group also recommends that the examiner complete

a training program that includes the analysis of low and high explosives,

instruction in the use of instrumentation used in routine analyses, the con-

struction of explosive devices, and participation in a postblast investigation

course. Although there is no official certification program for explosives

examiners, TWGFEX has devised a suggested training guide. The guide is

divided into seven modules, each with a reading list, practical exercises,

and methods of evaluation.¹¹³ To ensure that examiners maintain a level

of competency, proficiency testing (internal or external) is required by AS-

CLD/LAB once per calendar year.¹¹⁴

The ultimate goal of an explosives examination is the identification of

the explosive material used, whether it is through the analysis of an intact

material or of the residue left behind when the material explodes. Intact

material lends itself to being more easily identified. The individual compo-

nents of postblast residue may often be identified (e.g., potassium chloride

and potassium sulfate). The training and experience of examiners allows

them to deduce what types of explosive material were originally present

from possible combinations of explosive materials.

Whether it is a low explosive or high explosive, the analysis of an intact

explosive material follows a procedure that begins with a macroscopic and

microscopic examination of the material, followed by a burn test, when ap-

propriate. The results of the initial observations will dictate how the rest of

the analysis will proceed. Typically it will involve the use of instrumentation

that provides both elemental and structural information about the material,

such as X-ray diffraction, scanning electron microscope-energy dispersive

X-ray analysis, or infrared spectroscopy. TWGFEX has devised guidelines

for the analysis of intact explosives that categorize the instruments that can

be used based on the level of information they provide.¹¹⁵ The information

gathered, if sufficient, can be useful in identifying the material.

The analysis of postblast explosive residues begins much like the analy-

¹¹² TWGFEX Explosive Examiners Job Description. Undated. Available at http://ncfs.ucf.

edu/twgfex/documents.html.

¹¹³ TWGFEX Training Guide for Explosives Analysis Training. Undated. Available at http://

ncfs.ucf.edu/twgfex/Documents.html.

¹¹⁴ American Society of Crime Laboratory Directors International. 2006. Supplemental

Requirements for the Accreditation of Forensic Science Testing Laboratories, p. 20. See www.

ascld-lab.org/international/indexinternational.html.

¹¹⁵ TWGFEX Recommended Guidelines for Forensic Identification of Intact Explosives.

Undated. Available at http://ncfs.ucf.edu/twgfex/documents.html.

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STRENGTHENING FORENSIC SCIENCE IN THE UNITED STATES

sis of intact explosives, with the macroscopic and microscopic analysis of

the evidence submitted (whether it is an expended device, fragments of a

device, or debris from near the site of the explosion). If no intact explosive

material is found, a sequence of extracts may be used to capture any organic

and/or inorganic residues present. These extracts are then analyzed em-

ploying the same instrumentation used for intact explosives. However, the

results produced differ in their specificity, and it is here that the training and

expertise of the examiner plays a large role. To interpret the results prop-

erly, the examiner must have knowledge of the composition of explosives

and the reaction products that form when they explode. Interpretation can

be further complicated by the presence of contaminants from, for example,

the device or soil.¹¹⁶

Examination conclusions for postblast residues range from “the resi-

due present was consistent with an explosive material” to “the residue is

only indicative of an explosive” to “no explosive residues were present.”

TWGFEX recently has developed a set of guidelines for the analysis of

postblast explosive residues,¹¹⁷ but has yet to make any recommendations

for report wording.

The examination of fire debris not associated with explosions often

aims to determine whether an accelerant was used. To assess the effects of

an accelerant, one might design an experiment, under a range of conditions

(e.g., wind speed, temperature, presence/absence of other chemicals) with

two groups: one in which materials are burned in the presence of an accel-

erant (“treatment”) and one with no accelerant (“control”). The measured

outcomes on the burned materials might be measures that characterize the

damage patterns (e.g., depth of char, size of bubbles on surfaces). Differ-

ences in the ranges of these measurements from the materials in the two

groups (treatment versus control) suggest a hypothesis about the effects

of an accelerant. Following this exploration, one should design validation

studies to confirm that these measures do indeed characterize the differences

in materials treated or untreated with an accelerant.

Summary Assessment

The scientific foundations exist to support the analysis of explosions,

because such analysis is based primarily on well-established chemistry. As

part of the laboratory work, an analyst often will try to reconstruct the

bomb, which introduces procedural complications, but not scientific ones.

¹¹⁶ C.R. Midkiff. 2002. Arson and explosive investigation. In: R. Saferstein (ed.). Forensic

Science Handbook. Vol. 1, 2nd ed. Upper Saddle River, NJ: Prentice Hall.

¹¹⁷ TWGFEX Recommended Guidelines for Forensic Identification of Post-Blast Explosive

Residues. 2007. Available at http://ncfs.ucf.edu/twgfex/action_items.html.

FORENSIC SCIENCE DISCIPLINES

173

By contrast, much more research is needed on the natural variability

of burn patterns and damage characteristics and how they are affected by

the presence of various accelerants. Despite the paucity of research, some

arson investigators continue to make determinations about whether or not

a particular fire was set. However, according to testimony presented to the

committee,¹¹⁸ many of the rules of thumb that are typically assumed to

indicate that an accelerant was used (e.g., “alligatoring” of wood, specific

char patterns) have been shown not to be true.¹¹⁹ Experiments should be

designed to put arson investigations on a more solid scientific footing.

FORENSIC ODONTOLOGY

Forensic odontology, the application of the science of dentistry to the

field of law, includes several distinct areas of focus: the identification of

unknown remains, bite mark comparison, the interpretation of oral injury,

and dental malpractice. Bite mark comparison is often used in criminal

prosecutions and is the most controversial of the four areas just mentioned.

Although the identification of human remains by their dental characteristics

is well established in the forensic science disciplines, there is continuing

dispute over the value and scientific validity of comparing and identifying

bite marks.¹²⁰

Many forensic odontologists providing criminal testimony concerning

bite marks belong to the American Board of Forensic Odontology (ABFO),

which was organized in 1976 and is recognized by the American Academy

of Forensic Sciences as a forensic specialty. The ABFO offers board certifi-

cation to its members.¹²¹

Sample Data and Collection

Bite marks are seen most often in cases of homicide, sexual assault,

and child abuse. The ABFO has approved guidelines for the collection of

evidence from bite mark victims and suspected biters.¹²² The techniques

for obtaining bite mark evidence from human skin—for example, various

forms of photography, dental casts, clear overlays, computer enhancement,

electron microscopy, and swabbing for serology or DNA—generally are

¹¹⁸ J. Lentini. Scientific Fire Analysis, LLC. Presentation to the committee. April 23, 2007.

Available at www7.nationalacademies.org/stl/April%20Forensic%20Lentini.pdf.

¹¹⁹ NFPA 921 Guide for Explosion and Fire Investigations, 2008 Edition. Quincy, MA:

National Fire Protection Association.

¹²⁰ E.g., J.A. Kieser. 2005. Weighing bitemark evidence: A postmodern perspective. Journal

of Forensic Science, Medicine, and Pathology 1(2):75-80.

¹²¹ American Board of Forensic Odontology at www.abfo.org.

¹²² Ibid.

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STRENGTHENING FORENSIC SCIENCE IN THE UNITED STATES

well established and relatively noncontroversial. Unfortunately, bite marks

on the skin will change over time and can be distorted by the elasticity of

the skin, the unevenness of the surface bite, and swelling and healing. These

features may severely limit the validity of forensic odontology. Also, some

practical difficulties, such as distortions in photographs and changes over

time in the dentition of suspects, may limit the accuracy of the results.¹²³

Analyses

The guidelines of the ABFO for the analysis of bite marks list a large

number of methods for analysis, including transillumination of tissue,

computer enhancement and/or digitalization of the bite mark or teeth, ste-

reomicroscopy, scanning electron microscopy, video superimposition, and

histology.¹²⁴ The guidelines, however, do not indicate the criteria necessary

for using each method to determine whether the bite mark can be related

to a person’s dentition and with what degree of probability. There is no

science on the reproducibility of the different methods of analysis that lead

to conclusions about the probability of a match. This includes reproduc-

ibility between experts and with the same expert over time. Even when

using the guidelines, different experts provide widely differing results and

a high percentage of false positive matches of bite marks using controlled

comparison studies.¹²⁵

No thorough study has been conducted of large populations to estab-

lish the uniqueness of bite marks; theoretical studies promoting the unique-

ness theory include more teeth than are seen in most bite marks submitted

for comparison. There is no central repository of bite marks and patterns.

Most comparisons are made between the bite mark and dental casts of an

individual or individuals of interest. Rarely are comparisons made between

the bite mark and a number of models from other individuals in addition to

those of the individual in question. If a bite mark is compared to a dental

cast using the guidelines of the ABFO, and the suspect providing the dental

cast cannot be eliminated as a person who could have made the bite, there

is no established science indicating what percentage of the population or

subgroup of the population could also have produced the bite. This follows

from the basic problems inherent in bite mark analysis and interpretation.

As with other “experience-based” forensic methods, forensic odontol-

ogy suffers from the potential for large bias among bite mark experts in

evaluating a specific bite mark in cases in which police agencies provide

the suspects for comparison and a limited number of models from which

¹²³ Rothwell, op. cit.

¹²⁴ American Board of Forensic Odontology, op. cit.

¹²⁵ Bowers, op. cit.

FORENSIC SCIENCE DISCIPLINES

175

to choose from in comparing the evidence. Bite marks often are associated

with highly sensationalized and prejudicial cases, and there can be a great

deal of pressure on the examining expert to match a bite mark to a suspect.

Blind comparisons and the use of a second expert are not widely used.

Scientific Interpretation and Reporting of Results

The ABFO has issued guidelines for reporting bite mark comparisons,

including the use of terminology for conclusion levels, but there is no in-

centive or requirement that these guidelines be used in the criminal justice

system. Testimony of experts generally is based on their experience and

their particular method of analysis of the bite mark. Some convictions based

mainly on testimony by experts indicating the identification of an individual

based on a bite mark have been overturned as a result of the provision of

compelling evidence to the contrary (usually DNA evidence).¹²⁶

More research is needed to confirm the fundamental basis for the sci-

ence of bite mark comparison. Although forensic odontologists understand

the anatomy of teeth and the mechanics of biting and can retrieve sufficient

information from bite marks on skin to assist in criminal investigations and

provide testimony at criminal trials, the scientific basis is insufficient to

conclude that bite mark comparisons can result in a conclusive match. In

fact, one of the standards of the ABFO for bite mark terminology is that,

“Terms assuring unconditional identification of a perpetrator, or without

doubt, are not sanctioned as a final conclusion.”¹²⁷

Some of the basic problems inherent in bite mark analysis and inter-

pretation are as follows:

(1) The uniqueness of the human dentition has not been scientifically

established.¹²⁸

(2) The ability of the dentition, if unique, to transfer a unique pattern

to human skin and the ability of the skin to maintain that unique-

ness has not been scientifically established.¹²⁹

The ability to analyze and interpret the scope or extent of

distortion of bite mark patterns on human skin has not been

demonstrated.

ii. The effect of distortion on different comparison techniques is

not fully understood and therefore has not been quantified.

¹²⁶ Bowers, op. cit.

¹²⁷ American Board of Forensic Odontology, op. cit.

¹²⁸ Senn, op. cit.

¹²⁹ Ibid.

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STRENGTHENING FORENSIC SCIENCE IN THE UNITED STATES

(3) A standard for the type, quality, and number of individual charac-

teristics required to indicate that a bite mark has reached a thresh-

old of evidentiary value has not been established.

Summary Assessment

Despite the inherent weaknesses involved in bite mark comparison, it

is reasonable to assume that the process can sometimes reliably exclude

suspects. Although the methods of collection of bite mark evidence are

relatively noncontroversial, there is considerable dispute about the value

and reliability of the collected data for interpretation. Some of the key ar-

eas of dispute include the accuracy of human skin as a reliable registration

material for bite marks, the uniqueness of human dentition, the techniques

used for analysis, and the role of examiner bias.¹³⁰ The ABFO has devel-

oped guidelines for the analysis of bite marks in an effort to standardize

analysis,¹³¹ but there is still no general agreement among practicing forensic

odontologists about national or international standards for comparison.

Although the majority of forensic odontologists are satisfied that bite

marks can demonstrate sufficient detail for positive identification,¹³² no

scientific studies support this assessment, and no large population studies

have been conducted. In numerous instances, experts diverge widely in their

evaluations of the same bite mark evidence,¹³³ which has led to questioning

of the value and scientific objectivity of such evidence.

Bite mark testimony has been criticized basically on the same grounds

as testimony by questioned document examiners and microscopic hair ex-

aminers. The committee received no evidence of an existing scientific basis

for identifying an individual to the exclusion of all others. That same find-

ing was reported in a 2001 review, which “revealed a lack of valid evidence

to support many of the assumptions made by forensic dentists during bite

mark comparisons.”¹³⁴ Some research is warranted in order to identify

the circumstances within which the methods of forensic odontology can

provide probative value.

¹³⁰ Ibid.

¹³¹ American Board of Forensic Odontology, op. cit.

¹³² I.A. Pretty. 2003. A Web-based survey of odontologists’ opinions concerning bite mark

analyses. Journal of Forensic Sciences 48(5):1-4.

¹³³ C.M. Bowers. 2006. Problem-based analysis of bite mark misidentifications: The role of

DNA. Forensic Science International 159 Supplement 1:s104-s109.

¹³⁴ I.A. Pretty and D. Sweet. 2001. The scientific basis for human bitemark analyses—A

critical review. Science and Justice 41(2):85-92. Quotation taken from the abstract.

FORENSIC SCIENCE DISCIPLINES

177

BLOODSTAIN PATTERN ANALYSIS

Understanding how a particular bloodstain pattern occurred can be

critical physical evidence, because it may help investigators understand

the events of the crime. Bloodstain patterns occur in a multitude of crime

types—homicide, sexual battery, burglary, hit-and-run accidents—and are

commonly present. Bloodstain pattern analysis is employed in crime recon-

struction or event reconstruction when a part of the crime scene requires

interpretation of these patterns.

However, many sources of variability arise with the production of

bloodstain patterns, and their interpretation is not nearly as straightfor-

ward as the process implies. Interpreting and integrating bloodstain pat-

terns into a reconstruction requires, at a minimum:

• an appropriate scientific education;

• knowledge of the terminology employed (e.g., angle of impact,

arterial spurting, back spatter, castoff pattern);

• an understanding of the limitations of the measurement tools used

to make bloodstain pattern measurements (e.g., calculators, soft-

ware, lasers, protractors);

• an understanding of applied mathematics and the use of significant

figures;

• an understanding of the physics of fluid transfer;

• an understanding of pathology of wounds; and

• an understanding of the general patterns blood makes after leaving

the human body.

Sample Data and Collection

Dried blood may be found at crime scenes, deposited either through

pooling or via airborne transfer (spatter). The patterns left by blood can

suggest the kind of injury that was sustained, the final movements of a

victim, the angle of a shooting, and more. Bloodstains on artifacts such as

clothing and weapons may be crucial to understanding how the blood was

deposited, which can indicate the source of the blood. For example, a stain

on a garment, such as a shirt, might indicate contact between the person

who wore the shirt and a bloody object, while tiny droplets of blood might

suggest proximity to a violent event, such as a beating.

Analyses

Bloodstain patterns found at scenes can be complex, because although

overlapping patterns may appear simple, in many cases their interpreta-

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STRENGTHENING FORENSIC SCIENCE IN THE UNITED STATES

tions are difficult or impossible. ^135,136 Workshops teach the fundamentals

of basic pattern formation and are not a substitute for experience and ex-

perimentation when applying knowledge to crime reconstruction.¹³⁷ Such

workshops are more aptly applicable for the investigator who needs to

recognize the importance of these patterns so that he or she may enlist the

services of a qualified expert. These courses also are helpful for attorneys

who encounter these patterns in the course of preparing a case or when

preparing to present testimony in court.

Although there is a professional society of bloodstain pattern ana-

lysts, the two organizations that have or recommend qualifications are

the IAI and the Scientific Working Group on Bloodstain Pattern Analysis

(SWGSTAIN). SWGSTAIN’s suggested requirements for practicing blood-

stain pattern analysis are outwardly impressive, as are IAI’s 240 hours of

course instruction. But the IAI has no educational requirements for certifi-

cation in bloodstain pattern analysis.¹³⁸ This emphasis on experience over

scientific foundations seems misguided, given the importance of rigorous

and objective hypothesis testing and the complex nature of fluid dynamics.

In general, the opinions of bloodstain pattern analysts are more subjective

than scientific. In addition, many bloodstain pattern analysis cases are

prosecution driven or defense driven, with targeted requests that can lead

to context bias.

Summary Assessment

Scientific studies support some aspects of bloodstain pattern analysis.

One can tell, for example, if the blood spattered quickly or slowly, but some

experts extrapolate far beyond what can be supported. Although the trajec-

tories of bullets are linear, the damage that they cause in soft tissue and the

complex patterns that fluids make when exiting wounds are highly variable.

For such situations, many experiments must be conducted to determine

what characteristics of a bloodstain pattern are caused by particular actions

during a crime and to inform the interpretation of those causal links and

¹³⁵ H.L. MacDonell. 1997. Bloodstain Patterns. Corning, NY: Laboratory of Forensic

Science; S. James. 1998. Scientific and Legal Applications of Bloodstain Pattern Interpreta-

tion. Boca Raton, FL: CRC Press; P. Pizzola, S. Roth, and P. DeForest. 1986. Blood drop

dynamics-II. Journal of Forensic Sciences 31(1): 36-49.

¹³⁶ Ibid.; R.M. Gardner. 2004. Practical Crime Scene Processing and Investigation. Boca

Raton, FL: CRC Press; H.C. Lee; T. Palmbach and M.T. Miller. 2005. Henry Lee’s Crime Scene

Handbook. Burlington, MA: Elsevier Academic Press, pp. 281-298.

¹³⁷ W.J. Chisum and B.E. Turvey. 2007. Crime Reconstruction. Burlington, MA: Elsevier

Academic Press.

¹³⁸ See “Bloodstain Pattern Examiner Certification Requirements.” Available at theiai.org/

certifications/bloodstain/requirements.php.

FORENSIC SCIENCE DISCIPLINES

179

their variabilities. For these same reasons, extra care must be given to the

way in which the analyses are presented in court. The uncertainties associ-

ated with bloodstain pattern analysis are enormous.

AN EMERGING FORENSIC SCIENCE DISCIPLINE:

DIGITAL AND MULTIMEDIA ANALYSIS

The analysis of digital evidence deals with gathering, processing, and

interpreting digital evidence, such as electronic documents, lists of phone

numbers and call logs, records of a device’s location at a given time, e-

mails, photographs, and more. In addition to traditional desktop and lap-

top computers, digital devices that store data of possible value in criminal

investigations include cell phones, GPS devices, digital cameras, personal

digital assistants (PDAs), large servers and storage devices (e.g., RAIDS and

SANS), video game consoles (e.g., PlayStation and Xbox), and portable

media players (e.g., iPods). The storage media associated with these devices

currently fall into three broad categories. The first, magnetic memory, in-

cludes hard drives, floppy discs, and tapes. The second, optical memory,

includes compact discs (CDs), and digital versatile discs (DVDs). The third,

electrical storage, includes USB flash drives, some memory cards, and some

microchips. These items are the most commonly encountered in criminal

and counterintelligence matters, but laboratories have been asked to ex-

amine such items as scuba dive watches in death investigations and black

boxes in aircraft mishaps.

The proliferation of computers and related devices over the past 30

years has led to significant changes in and the expansion of the types of

criminal activities that generate digital evidence. Initially, computers were

either the weapon or the object of the crime. In the early days, most com-

puter crime involved manipulating computer programs of large businesses

in order to steal money or other resources. As computers became more

popular, they became storage containers for evidence. Drug dealers, book

makers, and white collar criminals began to keep computerized spread-

sheets detailing their transactions. Digital cameras and the Internet have

made child pornography increasingly available, and computers act as a

digital file cabinet to hold this contraband material. Finally, digital media

have become witnesses to daily activities. Many individuals have two cell

phones with text messaging and/or e-mail capability, several computers, a

home alarm system, a GPS in the car, and more; even children often possess

some subset of these items. Workplaces use magnetic card readers to permit

access to buildings. Most communication involves some kind of computer,

and by the end of each day, hundreds of megabytes of data may have been

generated about where individuals have been, how fast they got there, to

whom they spoke, and even what was said. Suicide notes are written on

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STRENGTHENING FORENSIC SCIENCE IN THE UNITED STATES

computers. Sexual predators stalk their victims online via e-mail, chat, and

instant messaging. Even get-away cars are equipped with GPS devices. Fi-

nally, computer systems have become (with ever-increasing frequency) the

victims of unauthorized control or intrusions. These intrusions often result

in the manipulation of files and the exfiltration of sensitive information. In

addition, computers in automobiles that track speed, breaking, and turn-

ing are valuable in accident reconstruction. As a result, almost every crime

could have digital evidence associated with it.

Sample Data and Collection

The best practices for the collection of digital evidence most often

call for the person at the scene to disconnect the power cord for the com-

puter and related peripheral equipments (e.g., monitor, printer) and seize

these items, as well as any loose storage media such as thumb drives and

CDs. This method works well in most cases. However, some data (like

recently typed passwords, malicious programs, and active communication

programs) are volatile and are stored in the electronic chips of the system.

In these circumstances, this information is lost when the device is turned

off. In intrusion investigations or in cases in which encryption software is

being used, this volatile information could be the key to a successful analy-

sis and prosecution.¹³⁹

Recognizing potential sources of digital evidence is also an ongoing

challenge. Investigators are likely to seize a desktop computer but walk past

a PlayStation. Thumb drives can be fashioned to look like a pocket knife,

writing pen, or even a piece of sushi. Cell phones and wireless Internet

capability present another challenge: If these devices are turned on while

in law enforcement custody, they could be remotely accessed and altered

by a suspect.

Analyses

The typical approach to examining a computer involves two main

phases. The first is the imaging phase. During this process, the storage

device (most often a hard drive) is fitted with an appliance that prevents

any new information from being written. Then, all of the data are copied

to a new blank hard drive. The copy is compared with the original, most

often by using a mathematical algorithm called Message Digest-5, other-

wise known as MD5 Hash. The MD5 Hash value gives a unique series of

numbers and letters for every file. In the examination phase, this forensi-

¹³⁹ See W.G. Kruse and J.G. Heiser. 2001. Computer Forensics: Incident Response Essen-

tials. Boston: Addison-Wesley.

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