In the new drug development process, how to correctly assess treatment efficacy in subgroups and their combinations can be nontrivial. It depends on the nature of efficacy measure as well as the estimation procedure. The current statistical practice of estimating the treatment efficacy in a mixture population has serious flaws. We propose a subgroup mixable estimation principle which respects the logical relationships between treatment efficacy in subgroups and their combinations. Focusing on the time-to-event outcomes and ordinal biomarkers, we develop a simultaneous inference procedure, with appropriate efficacy measures, to correctly infer treatment efficacy in a mixture population.