Application of trial inclusion/exclusion criteria, together with other factors affecting recruitment, may mean that the trial population does not coincide with the population likely to be treated locally in usual practice. There may be differences in the distribution by age, gender, ethnicity, socio-economic factors, co-morbidities or other factors. Firstly, this is of concern if the intervention of interest has different efficacy across these factors. Secondly, even in the absence of such efficacy differences, the risk profile of the study population for the outcome of interest may differ from that of the population likely to be treated in usual practice. In this case there may be different levels of absolute effect (events averted etc.) associated with levels of trial-reported efficacy for the different populations.