Major systematic bias can result when age-specific mortality data are sought through automated searches of large national death files, based on calculated odds in favor of a correct match. The bias occurs whenever the calculation fails to take into account: (a) the size of the death file being searched and (b) the age dependent likelihood that individual search record will find a correct match somewhere in that file. Indeed, there is widespread neglect of the "prior likelihood" or "prior odds" in favor of a fortuitous correct match on a single random pairing (i.e., before the identifiers have been compared). As a result, ratios of false matches to false nonmatches may be expected to vary widely in the different strata of a breakdown by age. A solution is described, using data from life tables and death statistics. The flexibility and merit of the "prior odds" for general application is emphasized.