Classification,
Identification, and Individualization
135
the analogous location on each piece rather than a potentially complementary
protrusion and recess), forcing the examiner to conclude that the paper
fragments must originate from different sources. The examiner bases the
decision on his knowledge of paper, information about this particular kind
of paper and its fragmentation qualities, experiments with tearing paper in
general and this paper specifically, how paper
degrades under various envi-
ronmental conditions, and information about the history of this sample. He
might try to recreate conditions that led to the kind of degradation seen.
Using all of this information in conjunction with his own personal experi-
ence, the analyst will either decide that the pieces of paper were once one
and any discontinuities are explainable or he will conclude that no reasonable
explanation exists for the boundary differences and that the paper fragments
originated from different intact sheets of paper. Similar considerations apply
to any physical match problem.
Biological evidence
. Some of the most physically fragile evidence is bio-
logical evidence. Once parted from a living system, biological
material quickly
begins to lose integrity. In a short period of time, even DNA, the most hardy
of physiological substances, degrades in both quality and quantity under
nonoptimal conditions. Because of this, the DNA profile from an evidence
sample may show intrinsic differences from a reference sample even if they
have a common origin. Fortunately, because of the scrutiny forensic DNA
typing has received, extensive validation studies have been performed, allow-
ing analysts to judge from a highly informed position
whether the differences
are explainable or unexplainable. For instance, the analyst may assess the
average size of the DNA in the sample before performing any analysis to get
an idea of what might be expected, and also to choose an analysis system
intelligently. If data for the largest locus are missing from an otherwise clear
evidence profile, and the assessment data predict that no DNA of a sufficient
size is present, then the absence of that particular datum is explainable.
Although its absence may conflict with a reference profile, this finding would
not by itself be cause to eliminate a common origin. Sometimes
the method
used to detect the evidence traits (an extrinsic factor) introduces apparent
differences in samples that do, in fact, originate from the same source. For
example, in DNA typing, differences in electrophoretic systems, such as the
presence or absence of denaturants, can produce apparently discrepant typing
results for the same sample (Fregeau and Fourney, 1993; Gill et al., 1994).*
Similarly, different PCR primer sets for the same STR locus may occasionally
* Differences in the resolution of an electrophoretic gel system may cause a locus that is
resolved as a heterozygote in one system to appear as a homozygote in another system.
Denaturants such as formamide or urea are used to increase
the resolution of certain
electrophoretic systems.
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136
Principles and Practice of Criminalistics
produce different results for the same sample (Kline and Jenkins, 1998; Walsh,
1998).*
Print and impression evidence.
It is interesting to note that almost every
aspect of print and impression evidence is extrinsic to the actual source
pattern. The transfer medium influences the fidelity of trait transfer and also
adds its own characteristics. The nature of the substrate onto which a two-
dimensional print is deposited, or the substance through which it is depos-
ited, may either obscure or reveal details of the source. For latent prints, the
substrate also affects
the choice of detection method, which adds yet another
layer to the visualization and resolution of the traits. Because the deposition
of any print or impression occurs in three-dimensional space, the angle of
its deposition will affect trait transfer, as will the force with which the impres-
sion is made. When examining an evidence impression or print, the crimi-
nalist must decide which traits are faithful to the source and which are
ambiguous as a result of these many possible extrinsic influences. Even two
reference dermal
ridge prints, for example, rolled under the most optimal
conditions, will show clear differences. Yet everyone agrees that differences
due to ink quality, ink quantity, transfer pressure, and transfer angle are not
genuine differences. In a bloody shoeprint, for example, three components
interact to create the print: the sole of the shoe, the blood, and
the substrate
(a floor or some other surface). Each will contribute elements to the final
quality of the evidence print. Uneven distribution of blood on the shoe sole
might mean that some traits are not transferred. A large volume of blood on
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