The
main purpose of this exercise is to show that the method of choosing a crime
scene has a large effect on the certainty of an identification. It might be
interesting to compare it with real fingerprint work.
10,000
crime scenes represents the whole of fingerprinting activity in an area. I
would imagine that in Scotland or the UK we will get up to that number in a few
years. In this exercise the whole town
was treated as suspects. In real fingerprint work most of the time is spent on
eliminations, where the latent prints of the victim and people who have a
legitimate explanation for being at the crime scene are eliminated from the
inquiry. Eliminations account for the
vast majority of all the identifications.
In an investigation with only one site a misidentification during
elimination will result in a latent being erroneously eliminated but it is not
likely to result in a false accusation. In investigations with multiple sites a
person on the elimination list would be thought to be innocent at one location
but could become a suspect if he or she was identified (or misidentified) at
another location.
400
latent prints in each house was chosen because that is the number of latents in
the Marion Ross murder house. The
database of 25,000 records is a modest example. The FBI's Criminal Master File contains 50 million although the
big national and international databases may contain a built-in safety
factor. A random match error in a big
database is unlikely to identify someone with a connection to the crime. I can
imagine that this would lead to a rethink.
Of
course in an area such as Scotland over a period of years there will be a lot
more than 40 crimes. However, for the
purposes of calculating the safety of making accusations of lying when a
disputed latent is found during an investigation, and the person identified is
not accused of the original crime, it is not the original crimes that we need
to estimate, it is the number of hidden events that would result in the
identified person lying that occur in the 10,000 crime scenes.