8127/frame/fm

 
2.
Validation of Methods for Comparison Evidence
In the previous section on optimization, we alluded to some of the fundamental
differences between techniques in which the evidence interacts chemically or
physically with some substance or instrument to produce a detectable basis for
comparison and techniques in which the comparison is based on strict visual
observation (with or without magnification). Because validation addresses the
interaction between the samples and the test system, we are left with somewhat
of a void when we ask what might constitute validation for comparison evidence.
For all kinds of comparison evidence where visual and optical methods are used
directly (including microscopy), the instrument with which the evidence inter-
acts is the human brain. This is, by definition, personal to each examiner. The
nature of this kind of evidence demands, first of all, that a high level of training
and experience (“optimization”) be required of each individual examiner.
Nevertheless, although the data can only strictly pertain to the individual
doing the testing, most aspects of validation can be explored for comparison
evidence. For example, simulating the consequences of intentional environ-
mental exposure of various materials can be very useful in helping the exam-
iner appreciate explainable differences in two items that may, in fact, share
a common origin. Can the faded red cotton evidence fiber and the bright
red cotton reference fiber be classified as originating from a common source?
For impression evidence, “minima” and “maxima” of detectable characteris-
tics might be explored by taking an item with known characteristics and
impressing it in various substrates with a variety of known pressures. In
Sidebar 9, we refer to one example of a validation experiment involving
dermal ridge prints and blood spatter. The creative criminalist can certainly
think of many more such examples.