Comments on the statistical aspects of the NRC's report on DNA typing

The goal of the NRC report on DNA typing was to answer a "crescendo of questions concerning DNA typing," many of them in the areas of population genetics and statistics. Unfortunately, few of these questions were answered adequately. In lieu of answering these questions, the panel proposed another conservative method of forensic inference, the "ceiling principle." Aside from its extreme conservativeness, this new method is difficult to justify because it is based on inadequate population genetics and statistical theory. Moreover, in its ultimate implementation, the panel's method will depend on a population genetics study whose rationale is questionable. In this article, we elaborate some of the general comments we made about the NRC report in a recent article [1]. Specifically we cover three topics. First we question the statistical basis for the ceiling principle, showing that the empirical results that motivated the method are likely to be misinterpreted and showing, by power calculations, that the effects of population substructure cannot be substantial. Second, we show that the study design to determine "ceiling" allele frequencies has several undesirable statistical properties. Finally, we discuss the estimation of handling errors from the statistical perspective, a subject treated inadequately by the report.