In
normal fingerprint work the police drawing a cordon round a crime scene is
painting the target and they do this before firing the shot (the fingerprint
analysis). If fingerprinting has a very small error rate it would be very
unlikely (but not completely impossible) for a misidentification to occur
within any set of only a few hundred latent fingerprints that have a connection
to a crime. But as long as the error
rate is not zero some misidentifications will occur in the
millions of identifications produced by the whole of fingerprinting activity
over a period of years. These
misidentifications will turn up one day and if we assume that someone is lying
when they deny depositing a fingerprint, and it is not connected with any known
crime, then we are painting the target after firing the shot.
The
core point about this fallacy is if something can happen either by chance or by
an assignable cause, and you want to prove that an example of it didn’t just
happen by chance, you cannot pick the case because it comes to your
attention. For example if you first
notice that a village has an exceptionally high cluster of cancers and then you
suggest that pollution from a nearby factory is the cause, you must find other
proof that the factory is causing the cancers.
But if you start by having an idea that the type of pollution emitted
from the factory causes cancers then you carry out a survey of the local
village and find an exceptionally high rate of the disease, you can
use the low probability that this cluster could have happened by chance as part
of your proof that the factory is the cause. In fingerprinting most
identifications are not denied so one that is denied attracts attention.
A correct identification that is denied is caused by the criminal being in the
crime scene and lying about it. Many
misidentifications will be random in nature, wrongly identifying someone by
chance who cannot be linked to the crime under investigation.
A
more common definition of the fallacy is that you cannot use the same
information to both construct and test the same hypothesis. In a normal criminal case the hypothesis is
something like "the criminal has deposited a fingerprint at the crime
scene and will deny it". The
hypothesis is constructed before the fingerprint work starts from the knowledge
that a crime has occurred. The crime itself is not part of the hypothesis
because we know it has happened. In the
McKie case, the hypothesis is "Shirley McKie entered Marion Ross' house in
breach of police procedures and deposited a fingerprint". This is
constructed entirely from the fingerprint match between McKie and Y7 and
includes the "crime" of entering the murder scene. It is unusual to have the crime as part of
the hypothesis in fingerprinting. When David Asbury won his appeal the question
arises, "If Asbury did not kill Marion Ross then who did"?. When
Shirley McKie was found not guilty of perjury nobody asked the question "If
it wasn't McKie who entered the house without permission then who
did"?
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