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i.
Collection and preservation.
We cover the proper collection of evi-
dence in Chapter 8. We emphasize here simply that, after its detection and
recognition, evidentiary material must be collected with an eye toward dis-
cerning and preserving any potentially individualizing traits. The TV private
detective who pokes his pen into the barrel of a weapon to preserve the
fingerprints is a caricature of the more subtle problems inherent in recog-
nizing, collecting, and preserving evidence. A generalist approach that con-
siders which types of evidence may be most relevant and helpful in answering
the questions posed in the case is clearly superior. Eventually, suitable refer-
ence materials must also be collected to complete the comparison.
 
ii.
Examination
 
Selection of class and individualizing traits from the evidence item
Selection of traits for comparison may be the most challenging aspect of the
examination of evidence, for it incorporates the education, training, and
experience of the examiner into a 
 
gestalt
. It involves careful examination and
observation of the item for those traits that may classify it and potentially
individualize it (Nordby, 1992; Smith et al., 1993).
Under ideal circumstances, the analyst will examine the evidence for
classifying or individualizing traits before examining the reference item
(Smith et al., 1993). This mitigates prejudgment on the part of the examiner,
forcing her to consider the uncertainty of the origin of any trait found in the
evidence prior to comparison with a putative source. While some may argue
with this contention, it surely decreases the value of a trait if it is detected
first in the reference and then found in the evidence. Detecting a trait first
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Principles and Practice of Criminalistics
in the reference might instill a subconscious expectation of finding that trait
in the evidence, especially if we already harbor some belief that they should
match. Thus, prior examination of the reference introduces the risk that we
might simply complete a pattern to fit our expectation when we examine the
evidence
(Nordby, 1992).
Evidence traits may certainly be defined even in the absence of a specific
reference. Sexual assault samples are routinely examined in suspectless cases,
resulting in DNA profiles of the semen donor. These traits are used to search
a data bank of previous offenders with the aim of linking the evidence to a
source. Similar databases also exist for firearms and fingerprint evidence. In
all cases, the analyst is forced to determine relevant traits from the evidence
before searching a reference database, demonstrating the feasibility of this
procedure.
 
How many traits?
This is one of the magic questions in forensic science. A single randomly
acquired trait is rarely sufficient to individualize a particular type of evidence.
In firearms examinations, for example, one can find two or three matching
striae in a row from different weapons (Biassotti, 1959; Murdock and Biasotti,
1997; Nichols, 1997). Similarly, one can readily find two or three matching
fingerprint minutiae from different fingers. Therefore, it is important to
accumulate a body of work that determines the expected characteristics and
limits of the evidence type.* In firearms examinations, consensus criteria
exist regarding the number and quality of traits required to convince the
examiner of a common source (Murdock and Biasotti, 1997; Nichols, 1997).
For shoeprints on the other hand, greater emphasis is placed on the analyst’s
judgment regarding the kind of traits that are present rather than the number
of traits (Cassidy, 1980; Bodziak, 1990). A dichotomy exists in the fingerprint
community whether more emphasis should be placed on an absolute number
* Although the wording differs, criteria for individualization offered by practitioners in a
wide variety of disciplines share a common thread. Some examples are as follows.