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g.
Review Your Testimony
As painful as it is, regular review of your testimony can be very helpful in
improving it. Most laboratories have a testimony monitoring program where
a supervisor or one of your peers provides a critique after listening to live
testimony. When transcripts are available, reading over your own testimony
can enlighten you in terms of what you said as opposed to what you thought
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Principles and Practice of Criminalistics
you said. In a situation, such as a deposition, where you are required actually
to sign the transcript, it would be remiss not to review the document thor-
oughly before signing it to make sure your words were transcribed correctly.
B.
Expressing Opinions — How Far Should 
the Analyst Go?
Throughout this book, we’ve related a number of concepts that justify the
expression of an opinion by a qualified expert.
1. A scientific hypothesis can never be proved; we can only fail to disprove
it (Popper, 1962).
2. In a Bayesian framework, we can compare the relative likelihood of
competing hypotheses.
3. A conclusion of individuality can never be proved; we can only become
convinced of it (Stoney, 1991).
4. A forensic scientist with appropriate education, training, and experi-
ence is the person best qualified to interpret scientific evidence, form
a conclusion about it, and provide an opinion about its significance
in the context of the case.
5. The law provides for the introduction of opinion evidence based on
inferences by qualified experts (
Federal Rules of Evidence
).
The criminalist is not only justified in rendering an opinion, he is obli-
gated to do so. This will commonly be an opinion concerning source deter-
mination, but, when appropriate, the qualified examiner may also speak to
an inference of contact (association) or reconstruction. Because we provide
a service to a group of people who are unable to understand our results for
themselves, we must interpret the results for them and form a conclusion,
often expressed in the form of an opinion. In simplifying the examination
or analysis such that its import can be easily understood by the laypersons
who fulfill the roles of counsel, judge, and jury, some technical accuracy is
necessarily lost. Throughout this book, we have discussed how the forensic
scientist must translate the legal question into a scientific question that can
be answered by examining or analyzing physical evidence. Finally, the process
must be reversed. The expert proffering the opinion is responsible for trans-
lating the scientific answers back into a form that is useful and understand-
able by the trier of fact.
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