A fresh look at proportional mortality ratios
This is a review of the use of proportional mortality ratios (PMRs). District health authorities rarely concern themselves with searching for unknown causes of disease but do need to monitor the health of a population. PMRs are a good way to do this. Textbooks of epidemiology tend to see PMRs as biased measures of risk and give them little attention. This review shows that the bias is small and of no practical importance. PMR studies can be seen as case-control studies and the major bias in these studies as analogous to Berkson's fallacy. We can then use the same techniques to reduce bias in PMR studies that are used in case-control studies. These are the use of several controls, the use of positive and negative controls, and the use of only one type of death in the denominator, rather than all causes of death. This article reviews these means of minimising bias using examples to show that careful selection of controls can overcome many of the supposed problems of PMR studies. It also shows how PMRs can control confounding in a way that SMRs cannot. PMR studies should be more widely used to monitor the health of the population.