Modelling multiple outcomes to improve the detection of causal mediation effects in complex intervention trials

Aim In a trial to increase physical activity in sedentary adults, there was no evidence of effect on the primary outcome, though large significant effects amongst eight related SF36 measures of general health [1]. How could such differences arise? Could they be: a true effect mediated through receiving the intervention, another systematic effect such as self-reporting bias, and/or chance? The aim was to develop reliable methods to investigate whether effects of the intervention on the SF36 outcomes were mediated through truly receiving the intervention delivered in intervention sessions.