  Unblinded sample size re-estimation (SSR) is often planned in a clinical
trial when there is large uncertainty about the true treatment effect. For
Proof-of Concept (PoC) in a Phase II dose finding study, contrast test can be
adopted to leverage information from all treatment groups. In this article, we
propose two-stage SSR designs using frequentist conditional power and Bayesian
posterior predictive power for both single and multiple contrast tests. The
Bayesian SSR can be implemented under a wide range of prior settings to
incorporate different prior knowledge. Taking the adaptivity into account, all
type I errors of final analysis in this paper are rigorously protected.
Simulation studies are carried out to demonstrate the advantages of unblinded
SSR in multi-arm trials.
