In
this exercise each of the 4 million latents was compared against all 25,000
people in the database so 100 billion (100 thousand million) comparisons were
carried out. It only needs one error in any of the 100 billon comparisons to
create a misidentification.
Fingerprint
experts have to avoid a 'random match error'.
This is where the fingerprint of someone who did not leave the latent
looks so similar to the person who did that an error happens. To achieve only 4 misidentifications our
fingerprint experts have to achieve a random match error of 1 in 25 billion.
This is 4 times the population of the world.
There
are other reasons for a misidentification such as the fingerprint examiner
knowing that they are checking the print of someone who lives in the house or a
near neighbour. This knowledge could
make them subconsciously biased towards an identification. In science, results are mistrusted unless steps
have been taken to make bias impossible.