DNA profiling: A discussion of issues relating to the reporting of very small match probabilities

DNA profiling has brought to the courts a new way of looking at forensic sc ience evidence. The weight of evidence, where there is a match between the profiles of a defendant and a crime sample, is presented int he form of a m atch probability. In all other areas of forensic science, it is long accept ed practice for the scientist to give an opinion of the form "in my opinion , x and y have the same source" but recent judgments have established that this is not to be permitted when x and y are DNA profiles. Yet DNA profilin g is better understood from a statistical standpoint than any other forensi c techniques, including fingerprints and, as profiling techniques become mo re powerful, so the match probabilities can be expected to become smaller. This paper discusses issues relating to how such probabilities should be pr esented at court.