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Principles and
Practice of Criminalistics
Probably the most contentious term is the word
match
. Interestingly,
Webster’s
(1996) lists no fewer that 24 definitions for “match.” Two (the first
and third) are of particular interest to forensic science.
1. A person or thing that equals or resembles another in some respect.
2. A person or thing that is an exact counterpart of another.
Figure 11.4
DNA Advisory Board standards for reports and review.
(From
Sci-
entific Working Group for DNA Analysis Methods (SWGDAM),
available at
http://www.forswg.org/swgdamin.htm
.)
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Communicating Your Results — Where Science Meets the Law
283
It is the difference between these two statements that forms the basis of the
confusion. It is technically correct, in our opinion, to say that the results of
all
tests done on two items match, or that two patterns match. This relies on
the first definition listed above. The point, of course, is that we obviously
cannot know if the two items match for some attribute that was not tested.
It is therefore incorrect and potentially misleading to say that, based solely
on the tests that were run, that the two
items
match. This
statement is the
second one listed above. One might make this inference, incorporating some
assumptions in the process, or become convinced of it and form an opinion.
However, because we, as scientists, have not explicitly stated which definition
we mean to imply, and the word, as
commonly used, has multiple definitions,
the situation simply begs for misunderstanding and misuse. In particular,
many laypersons easily equate “match” with “unique common origin.” This
may or may not be what the forensic scientist means to convey.
Another widely used term, and one that is equally successful at fomenting
debate, is the phrase
consistent with
.
Webster’s
(1996) gives the following as
one definition of “consistent”:
1. Agreeing or accordant; compatible; not
self-contradictory
This phrase is highly ambiguous because it encompasses such extremes of
meaning that the reader could easily be misled regarding the strength of an
inference. This is reflected in the weak and nonspecific common understand-
ing of the term, as described above. For example, an evidence sample of type
O blood is “consistent with” coming from an individual with type O blood.
It is also “consistent with” coming from any other person in the 40% of the
population that has type O blood. In contrast, an
evidence sample typed at
15 RFLP loci and 13 STR loci is also “consistent with” coming from an
individual who shows the same type at all 28 loci. The calculated frequency
of this type is so rare, however, that it is unreasonable to believe that it
describes more than one person on earth. Using “consistent with” to describe
both of these situations potentially overstates the blood-typing results and
also understates the DNA result. As a consequence, the
layperson reading the
report or hearing the testimony may give much more weight to the blood
typing result than would be appropriate, and conversely, underestimate the
strength of the DNA result. This is particularly true if no additional infor-
mation is presented in the report to describe the strength of the inference.
We do not find this phrase to be useful in communicating the results of a
forensic analysis.
A term that we do find useful is
indistinguishable
. In part, because it is a
six-syllable word and
not common in everyday speech, it forces the reader to
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actually think about its meaning. Combined with the appropriate limitations,
such as “indistinguishable by the tests performed” or “at this particular level of
analysis,” and a brief description of the tests and their limitations, this term can
provide the reader or listener with a good feeling for the strength of the result.
We are not under the illusion that terms such as “match” and “consistent
with” will
disappear from forensic usage, nor do we believe that “indistin-
guishable” is the sole best solution to the dilemma of accurately describing
our results. We do invite practitioners to think about the language that they
use to describe their conclusions about a forensic analysis and suggest that
the various professional groups might begin discussions about adopting a
standard terminology. This would also assist attorneys to better understand
the strengths and weaknesses of the physical evidence in the context of the
case, and the trier of fact to decide how much weight
to give to a particular
piece of evidence.
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