Sample sizes for event rate equivalence trials using prior information.

Trials for demonstrating the 'equivalence' of active standard and test treatments generally require large sample sizes that depend on the definition of 'equivalence' and the overall event rate when the outcome is incidence of an event such as mortality. The planning of sample sizes for such trials requires specification of a value for the overall event rate. This value often will reflect the outcomes of previous trials of the standard treatment, and is subject to uncertainty that needs some accommodation, to protect against an inadequate sample. Bayes and Empirical Bayes methods can be used to incorporate information from one or more previous trials into the sample size calculation when equivalence means high confidence that the event rate ratio is less than some specified value.