Estimating the Magnitude of InteractionbyDennis

Qualitative interaction (Peto, 1]) is a useful concept, but its practical value is limited because its presence is hard to detect. This diiculty arises from trying to detect a treatment eeect of opposite direction to the main eeect in a small number of subgroups. Simulation results are presented which reinforce this point. A new measure of interaction magnitude, IM, is introduced and proposed as an alternative way to think about important interaction between treatments and patient subgroups. Simulation results show that comparing a lower bound for IM to a speciic cutoo provides greater power to detect important interaction than do the tests for qualitative interaction.