There are at least two ways to address deviations. First, we should consider redesigning the intervention to eliminate deviations. Second, if important deviations cannot be eliminated, we should consider redesigning the intervention to improve its robustness against those deviations. Consider, for example, an intervention that consists of six consecutive sessions that build on each other. When the intervention is implemented in practice settings, many patients might skip some sessions, which might result in poor outcomes. If a high level of adherence cannot be achieved, we might redesign the intervention to make the sessions less dependent on each other—for example, by including redundant material. We could then conduct an efficacy study of the redesigned intervention to assess the impact of nonadherence by varying experimentally the number of sessions attended and the level of dependence between sessions. We might find that the redesigned intervention is less beneficial than the original intervention of all six sessions but more beneficial than the original was when some sessions were skipped. Under this scenario, the redesign has improved the robustness of the intervention against nonadherence; it will also deliver better overall effectiveness if nonadherence is substantial.