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In commenting on the report of child injury league tables issued by UNICEF,1 Ramsay asked the rhetorical question: Do international comparison help?2 Chalmers and Pless gave a conservative reply in the Editorial in Injury Prevention—“we certainly hope so”.3 Unlike Langley, I would like to give a more affirmative reply: I am convinced that international comparisons do help and are in fact essential for avoiding type III error, that is giving the right answer for the wrong question.4
The UNICEF report concerned two kinds of questions: why were there large variations in child injury mortality among countries that are part of the Organisation for Economic Cooperation and Development? And why did changes in child injury mortality differ from 1971–75 to 1991–95 among these countries? However, by using traditional, individual based epidemiological research methods to answer these questions, we run the risk of committing the type III error. Rose cautioned that we need to distinguish between two kinds of aetiological questions: the first asks about the “causes of cases” whereas the second asks about the “causes of incidence”.5, 6
Schwartz and Carpenter extended Rose's argument, illustrating it by using the causes of stroke.7 The causes of interindividual variation in risk within a population (for example, diabetes, genetic vulnerability) may be distinct from the causes of differences in the disease rate over time (that is, global economic crisis) or between populations (that is, differences …
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