15 Power for treatment by subgroup interaction tests in clinical trials

Oregon Convention Center 

Few analytic results exist in the current literature for the power of interaction tests in the context of clinical trial design. The power for the interaction between the treatment and a binary subgroup indicator is computed for trials with continuous, time-to-event, or binary outcomes. Quantitative interactions are assumed, when the treatment effect is heterogeneous but positive in both subgroups. Conditional on parameters of the overall design, for normal outcomes the power for a subgroup interaction can be expressed as a function of the effect and sample size in the more positive subgroup. For quantitative interactions, when the more positive subgroup has a larger effect size, power for the interaction is low. When the more positive subgroup has a smaller sample size but a large treatment effect, power can be equal to or greater than the power for the overall effect, though this situation is probably not common. Better appreciation of the power for interaction tests in clinical trials may lead to a clearer understanding of related design issues and better trial design.