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1.
Conclusion, Inference, or Opinion of Common Source
The analyst must now express the results of his formal consideration of the
relationship between the evidence and reference items. He started the exam-
ination by formulating (explicitly or not) a source determination question
in the form of the null hypothesis: “I will prove that this reference is the
source of the evidence.” Failure to disprove the null hypothesis means that
the analyst 
hasn’t
shown that the evidence and reference 
do not
share a
common origin. In other words, the analyst is forced to admit that they may
share a common source, and, in the case of classified evidence, that the
evidence may also have other potential sources. A way of expressing this in
* Most often, the items are an evidence sample and a reference sample, but they could
also be two evidence items. 
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Principles and Practice of Criminalistics
a few words is that the analysis leads to an 
inference of common source between
the evidence and reference samples
. Note that both classified and individual-
ized evidence may lead to an inference of common source; the difference is
in the number of sources, one for individualization, or more than one for
classification.
Throughout this chapter, we have referred to an 
inference 
of common
source, rather than a
 conclusion
of common source. We have also used the
term 
opinion 
to summarize the individualization process. 
Webster’s
(1996)
defines these terms as follows:
We believe the term 
inference
is best suited to communicating our belief of
common source. For our purposes, an inference is more appropriately
defined as a conclusion derived from both facts 
and
assumptions. An infer-
ence is not fact per se; it is only as strong as the limitations of the combination
of evidence, test, and test results. An expression of these limitations will be
different for classified evidence than it is for individualized evidence. We
develop these ideas in the Section F of this chapter.