EDITOR—Liu et al used the term “proportional mortality study” to describe their method of comparing the smoking habits of 0.7 million adults who died of neoplastic, respiratory, or vascular causes with those of a reference group of 0.2 million who died of other causes in China.1 The term can be confusing as it is used only for proportional mortality ratio analysis in standard epidemiology textbooks.2 We suggest that the study can be more easily understood if it is described as a case-control mortality study.

An important assumption in such analyses is that the other causes of death should be unrelated to the exposure “not only in the sense of causation but also in terms of ‘self-selection’ for the exposure and the diagnosis and certification of the underlying cause of death.”3 Liu et al validated this assumption by showing that the smoking rates of the male and female reference groups were only slightly higher than those of the surviving spouses of the people who had died. However, they did not elaborate whether this similarity was true for each city or rural area in China, and, if it …