Bayes Factors: What They are and What They are Not

Bayes factors have been offered by Bayesians as alternatives to P values (or significance probabilities) for testing hypotheses and for quantifying the degree to which observed data support or conflict with a hypothesis. In an earlier article, Schervish showed how the interpretation of P values as measures of support suffers a certain logical flaw. In this article, we show how Bayes factors suffer that same flaw. We investigate the source of that problem and consider what are the appropriate interpretations of Bayes factors.