The widely used ‘person‐years method’ of calculating expected mortality has been discussed recently by several authors. In studies where mortality is either lower or higher than the standard mortality of some reference population, the use of exposure to death as an estimator of the expected number of deaths will generally lead to bias, always exaggerating the difference between study and standard mortality. This bias is examined in a proportional hazards model. The recent suggestion by Hartz et al.¹ of calculating the mortalities of individuals during their ‘potential follow‐up time’ is claimed to be only rarely feasible in practice.