In clinical drug development a typical phase three power calculation for a
Go/No-Go decision is performed by replacing unknown population-level quantities
in the power function with what is observed from a literature review or what is
observed in phase two. Many authors and practitioners view this as an assumed
value of power and offer the Bayesian quantity probability of success or
assurance as an alternative. The claim is by averaging over a prior or
posterior distribution, probability of success transcends power by capturing
the uncertainty around the unknown true treatment effect and any other
population-level parameters. We use confidence distributions to frame both the
probability of success calculation and the typical power calculation as merely
producing two different point estimates of power. We demonstrate that Go/No-Go
decisions based on either point estimate of power do not adequately quantify
and control the risk involved, and instead we argue for Go/No-Go decisions that
utilize inference on power for better risk management and decision making. This
inference on power can be derived and displayed using confidence distributions