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c.
The Items Share a Unique Common Origin
The third possible outcome, that the items share a unique common origin,
represents the culmination of the individualization process. Individualization
provides information about the relationship of the evidence to a source. If
the two items exhibit the same individualizing traits in sufficient number or
quality, the analyst infers that they share a common origin to the exclusion
of all others. The evidence and reference items have been individualized to
a unique source. Always bear in mind that the analyst becomes 
convinced
of
individuality (Stoney, 1991a). The characteristics seen are so rare that it is
unreasonable to believe that they could be duplicated by chance in another
similar object. No evidence to the contrary would change the mind of the
examiner about the source of the item.
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Principles and Practice of Criminalistics
d.
The Conclusion
The analyst must provide a 
conclusion
that summarizes the strength of the
evidence so that the full import of the inference is communicated. This will
include (1) a statement inferring a common source, (2) a qualifying statement
that portrays whether the analyst believes that common source to be, in his
opinion, the only possible source, or whether multiple potential sources exist,
and (3) the likelihood of encountering other potential sources in this case as
a random chance.
F.
Mathematical Approaches to Expressing the Strength 
of the Inference
In the first part of this chapter, we discussed different possible inferences
about the relationship of evidence and reference samples. If an evidence
sample cannot be excluded as originating from the same source as a reference
sample, we infer either that multiple sources are possible (
classification
) or
that they share a single common source (
individualization
). We spend the
rest of this chapter exploring the quantitative expression of this relationship
of evidence to source.
In the preceding chapters, we have alluded to the discrepancy both in
the availability and expectation of quantitative data for different evidence
types. We make no secret of our proclivity toward the use of quantitative
data to assess the link between evidence and reference for class evidence, and
to support an opinion of a single common source for individualizing evi-
dence. However, we also acknowledge the difficulties inherent in acquiring
and applying frequency data for nonbiological evidence, some of which we
discuss later in this chapter. At the very least, however, we feel that all forensic
scientists should familiarize themselves with the various mathematical tools
available to help assess the strength of the evidence. The rest of this chapter
is devoted to introducing some of these tools.