Selection effects and database screening in forensic science

We argue that it is, in principle, not difficult to deal with selection effects in forensic science. If a suspect is selected through a process that is related to the forensic evidence, then the strength of the evidence will be compensated by very small prior odds. No further correction is necessary. The same is true for so-called data-dependent hypotheses. These are allowed, since if the hypothesis is really “tailored around” the evidence, the evidential value will be high but the prior odds will compensate for that. The assessment of the prior odds is outside the scope of the forensic scientist, but he should make lawmakers, judges and juries aware of the phenomenon. This discussion applies to many situations—we discuss four concrete examples.