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SUMMARY Deoxyribonucleic acid profiling has attracted widespread publicity because of the impact that it is making on the investigation of crime. Whereas considerable effort has been expended on the refinement of the laboratory systems for carrying out the technique, the development of efficient numerical procedures for evaluating the evidence contained in the profiles has attracted little attention. We have developed a method, based on the Bayesian likelihood ratio, for evaluating fragment length data in a crime case where a profile from a suspect is to be compared with a profile from a sample taken from the scene of the crime. Our treatment takes account of correlation in fragment length measurement errors and avoids an independence assumption which is currently being made by practitioners. We describe experiments which demonstrate the superiority of the method over the conventional method which is based on simple hypothesis tests.

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We describe a series of experiments, carried out on data from DNA profiles, which have been designed specifically to test the validity of the statistical procedures currently used in the Home Office Forensic Science Service. The tests address issues which have been the subject of topical debate, in particular those of representativeness and band independence. The results confirm the confidence which had already been placed in the established procedures. We recommend that all practitioners in the forensic field should carry out similar testing on their own data collections.

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