8127/frame/fm

parts of the sole might push blood into the recesses of the shoe sole, produc-
ing a featureless deposition in that area and obliterating potentially individ-
ualizing traits. An uneven substrate, such as floor tile, will likely take the shoe
impression on the peaks of the surface but not in the valleys. Given a nice
even reference print with lots of detail, and an uneven, partial evidence print,
the two items are clearly distinguishable. Yet, a shoeprint examiner might be
perfectly justified in concluding that they originate from the same source.
This is one of the more extreme examples where determining which differ-
ences are explainable and which are not dominates the final conclusion.
Both the known characteristics and limits of the evidence and the capa-
bilities and limitations of the method, and any other factors extrinsic to the
evidence, must be acknowledged so that both concordant and discordant
traits can be accorded the proper relative weight. However, when a single
* A sample may contain a primer binding site mutation that affects a primer in one
amplification system but not another. The result for this particular sample is that in one
PCR amplification system, the sample appears homozygous for that locus, while in a second
system it appears heterozygous. 
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Classification, Identification, and Individualization
137
difference between an evidence and reference item is seen that cannot be
explained by the nature of the evidence or the nature of the test, then the
two items must be concluded to have a different source.
E.
Inferring a Common or Different Source
At the termination of the analysis and comparison processes, the examiner
considers three questions:
1. Do unexplainable differences exist that convince me that the items
compared* originate from different sources?
If not,
2. Do the quantity and quality of the class characteristics convince me
that the items could originate from the same source?
If so,
3. Do a sufficient number and quality of individualizing traits exist to
convince me that they originate from the same unique source?
This opinion is the culmination of all of the collection, preservation, and
analytical work, and it relies on information accumulated by the field as well
as the knowledge, education, and experience of the analyst. A formal con-
sideration of all of the data requires the examiner to contemplate each of the
above two questions in turn. It should be clear what criteria were used, what
data were gathered to satisfy the criteria, and how they support the final
conclusion.