8127/frame/fm

 
(A)ssumptions + (F)acts = (I)nference
8127/frame/ch10 Page 264 Friday, July 21, 2000 11:38 AM

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Good Forensic Practice — Obligations of the Analyst
265
to which two or more individuals may have contributed, that engenders lively
debates. A complete approach would be to state competing hypotheses based
on different (reasonable) assumptions regarding how many individuals, or
which of the principals, might have contributed to the sample, and follow
each through to a logical conclusion. An example from 
 
Simpson
, detailed in
Sidebar 12 illustrates this process very well.
 
b.
Explainable Differences
In Chapter 6, we discussed at great length the idea of explainable differences,
and mechanistically, how evidence and reference samples might diverge once
they are separated. Briefly, any differences seen might be due to actual dif-
ferences inherent in the evidence (
 
intrinsic
 
differences
) or they might be a
consequence of the imprecision of the test system (
 
extrinsic
 
differences
). First,
the forensic scientist must evaluate any differences between evidence and
reference to determine if they are significant and how they might be
explained. Differences will virtually always exist; it is the job of the criminalist
to decide how to interpret them. The role of the expert is to use all of
information to make an informed judgment. Without apology, we note that
this is a subjective process, and should only be undertaken by a scientist with
the proper education, training, and experience. Qualified experts may dis-
agree. It is crucial to appreciate that reference and evidence items need not
be, nor are expected to be, absolutely identical in every detectable character-
istic for a conclusion of common source. As Kirk (1953) cogently points out:
Moreover, the finger is not actually identical from moment to moment, but
its significant features that are useful for practical fingerprinting remain
permanent and unchanged.