Multiple secondary outcome analyses: precise interpretation is important

Analysis of multiple secondary outcomes in a clinical trial leads to an increased probability of at least one false significant result among all secondary outcomes studied. In this paper we question the notion that that if no multiplicity adjustment has been applied to multiple secondary outcome analyses in a clinical trial then they must necessarily be regarded as exploratory. Instead, we argue that if individual secondary outcome results are interpreted carefully and precisely, there is no need to downgrade our interpretation to exploratory. This is because the probability of a false significant result for each comparison, the per-comparison wise error rate, does not increase with multiple testing. Strong effects on secondary outcomes should always be taken seriously, and must not be dismissed purely on the basis of multiplicity concerns.