Байесовская статистика и ДНК-доказательства: ограничения допустимости в суде.

Ruling which criticised the use of Bayesian statistics in determining guilt
the 1957 case, upheld in Bland, that allows a doctor to hasten death with a drug if primary intention is properly the ease of suffering

R v Adams [1996] EWCA Crim 10 and 222, are rulings in the United Kingdom that banned the expression in court of headline (soundbite), standalone Bayesian statistics from the reasoning admissible before a jury in DNA evidence cases, in favour of the calculated average (and maximal) number of matching incidences among the nation's population. The facts involved strong but inconclusive evidence conflicting with the DNA evidence, leading to a retrial.

A rape victim described her attacker as in his twenties. A suspect, Denis Adams, was arrested and an identity parade was arranged. The woman failed to pick him out, and on being asked if he fitted her description replied in the negative. She had described a man in his twenties and when asked how old Adams looked, she replied about forty. Adams was 37; he had an alibi for the night in question, his girlfriend saying he had spent the night with her. The DNA was the only incriminating evidence heard by the jury, as all the other evidence pointed towards innocence.

The only evidence against Adams was the DNA evidence. His age was substantially different from that reported by the victim, the victim did not identify him and he had an alibi which was never disproved. The 1 in 200 million match probability calculation did not allow for the fact that the perpetrator might be a close relative of the defendant – an important point, since the defendant had a half brother in his 20s whose DNA was never tested.

The Court of Appeal after the appeal wrote the guidelines for the way that match probabilities should be explained to jurors. Judges should say something along the lines of the following. "Suppose the match probability is 1 in 20 million. That means that in Britain (population about 60 million) there will be on average about 2 or 3 people, and certainly no more than 6 or 7, whose DNA matches that found at the crime scene, in addition to the accused. Now your job, as a member of the jury, is to decide on the basis of the other evidence, whether or not you are satisfied that it is the person on trial who is guilty, rather than one of the few other people with matching DNA. We don't know anything about the other matching people. They are likely to be distributed all across the country and may have been nowhere near the crime scene at the time of the crime. Others may be ruled out as being the wrong sex or the wrong age group."