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8.
Navigating the Testimonial Wilderness — Pitfalls, Traps, 
and Ambushes
Although we cannot make a complete study of all the possible uncomfortable
situations in which you might find yourself as an expert witness, we highlight
a few common situations that arise with some regularity.
 
a.
Reasonable Certainty
Because the law works in absolutes, while science provides anything but, the
never-ending struggle is for scientists to render their opinion in a way that
is useful to the layperson in making a decision about guilt or innocence. In
medical testimony, the phrase 
 
to a reasonable medical certainty
is often used
to convey the idea that, although we can never positively prove that the victim
was killed by the third bullet, the pathologist’s expert opinion carries enough
weight that the trier of fact may accept it as true. From the medical model
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Principles and Practice of Criminalistics
has come the phrase 
 
to a reasonable scientific certainty
. Both the judicial
system and some experts have latched onto this phrase as a convenient way
to render an opinion as fact. As convenient as it might be, it is a 
 
nonsequitor
.
As we have repeatedly discussed throughout this book, the notion of scientific
certainty does not exist. In our opinion, scientific experts should refrain from
resorting to that phraseology in expressing their opinions.
 
b.
Know What You Did
You must be intimately familiar with your contributions to the case. Never
testify to what you think you would normally do in general practice if you
aren’t explicitly sure you did it in this case. In 
 
Simpson
, an expert testified
that he always wore gloves when handling evidence samples at a crime scene.
The defense followed that testimony with a videotape of the witness grasping
a piece of evidence with his bare hands (
 
People v. Simpson
, April 11). The
credibility of the witness was effectively destroyed by that simple demonstra-
tion of his apparent lack of appreciation for specifics.