EDITOR—Parry et al draw attention to the difficulties faced by those wishing to use comparative outcome data to indicate performance.1 They clearly show the importance of adjusting for differences in case mix and allowing for random variation by establishing 95% confidence intervals for estimates of adjusted outcome. In addition to the uncertainty in the observed mortality, however, there is uncertainty in the predicted mortality. The overall lack of clarity in the rankings of the neonatal intensive care units might therefore …