From Richard Graham
Linda Geddes states that the first time DNA evidence was challenged in a UK court was during the trial of Sean Hoey for the 1998 bombing in Omagh, Northern Ireland (21 August, p 8). While it is true that until then DNA evidence had not been substantially challenged on the basis of its scientific reliability, the way it was presented in court had previously been challenged successfully several times.
In the case of Regina v Deen 1993, for example, DNA evidence was challenged on the grounds of the “prosecutor’s fallacy”. This way of presenting evidence mistakenly implies that the probability that the accused is innocent must be small when their DNA has been found to match a sample at the crime scene, as the probability of a randomly chosen person’s DNA matching that from the crime scene is also small.
Forensic scientists and statisticians have expended much energy in developing the best way to calculate the statistics relating to DNA evidence. It is equally important that we understand how these statistics should be best presented in court.
Ashbourne, Derbyshire, UK
