It is never correct to claim that treatments have no effect or that there is no difference in the effects of treatments. It is impossible to prove a negative or that two treatments have the same effect. There will always be some uncertainty surrounding estimates of treatment effects, and a small difference can never be excluded.1

Claims of no effect or no difference may mean that patients continue to be denied or exposed to interventions with important effects, either beneficial or harmful. They may also suggest that further research is unnecessary, so delaying satisfactory estimates of treatment effects.

The impossibility of proving no effect or no difference should be distinguished from the concept used for equivalence trials, where bounds are set on the differences that are deemed practically important. An analysis of 45 reports of trials purporting to test equivalence …