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particularly with regard to quantitation of the data (Houck, 1999). To some
extent, this is due to the historical acceptance of these types of analyses in
court without having demanded a demonstration of rigorous scientific under-
pinning, including an articulation of interpretational guidelines and a statis-
tical frequency estimate. In particular, we refer to those examinations that
require no laboratory analysis, but “only” a visual comparison. Such exami-
nations include physical match comparison, microscopic comparison of hairs
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14
Principles and Practice of Criminalistics
and fibers, and print and impression evidence. Rarely will an analyst be asked
about the fingerprint database on which a conclusion of common source has
been based (Cole, 1998; 1999), or the number or proportion of microscopic
striae that must concord between an evidence and test bullet for the analyst
to conclude that they were fired from the same gun. Some argue that inherent
differences in the nature of each type of evidence preclude a universal statistical
or interpretational model for all physical evidence (Houck, 1999).
An evolving standard for the legal admissibility of scientific evidence has
prompted a review and closer scrutiny of all methods, some of which have
been accepted without question for the last century (Cole, 1998; 1999). While
DNA analysts (recent FBI pronouncements notwithstanding) are expected
to provide an estimated frequency of a genetic profile in the relevant popu-
lation, the print or firearms analyst simply renders an opinion that the
association between evidence and reference is unique. It has been suggested
that comparison evidence might fall within the purview of an acquired skill
or expert opinion. Therefore, examiners should be able to express opinions
of identity without having to provide numeric estimates or systematic scien-
tific justification (Ashbaugh, 1996; Murdock and Biasotti, 1997). Recent court
decisions (
 
Daubert
, 1993; 
 
Kumho
, 1999) that comment directly on the admis-
sibility and definition of scientific evidence, and to what standard it should
be held, have provoked this discussion anew among the forensic community.
Certainly, defining the elements of a valid shoe print or glass fracture
comparison presents a greater challenge than defining the elements of sim-
ilarity necessary for a DNA or solid dose drug analysis. This follows directly
from the nature of the evidence. Shoe wear and breaking glass generate highly
random and complex patterns that may actually require greater expertise, or
at least more experience, to interpret than the much simpler and more
constrained patterns found in a spectrum or autoradiograph. Certainly, some
criminalists debate whether any useful purpose would be served by demand-
ing a more scientific treatment of disciplines that have traditionally relied on
the experience and expertise of each individual examiner (Ashbaugh, 1996;
Murdock and Biasotti, 1997). In a 1997 editorial, John Thornton, one of the
leading forensic minds of our day, wrote, “To master statistical models to
explain much of our evidence may be a slow, reluctant march through enemy
territory, but we must begin to plan for that campaign” (Thornton, 1997).
We agree that, at least for physical evidence, providing a statistical justifica-
tion for the analyst’s opinion should be a goal. This may take different forms
for different evidence; it may be limited for certain types of evidence until
further work is done, and remain limited for some kinds of evidence due to
its nature. However, in our opinion, it is an objective that we should be
actively pursuing. The discussion within the forensic community remains
heated and current.
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Introduction
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