Professor Eiji Yano raises a number of issues in his letter¹ which responded to my commentary² on his article³ about the Japanese spousal study, as does Chapman in his editorial.⁴ Here I reply to the main points raised.

Studies of environmental tobacco smoke (ETS) exposure and lung cancer commonly identify a group of self reported non-smoking women and then compare risk according to the smoking habits of the husband. If some true smokers are erroneously included among the female subjects, an apparent relationship of spousal smoking with lung cancer may be seen even when no true effect of ETS exists. This has been mathematically demonstrated (for example, Lee and Forey⁵), with attempts to correct for it made by major independent authoritative reviews of the evidence on passive smoking and lung cancer.^6–8 The magnitude of the bias depends (among other things) on the extent to which women who smoke are misclassified as non-smokers. It can also be shown mathematically⁵ that a given rate of misclassification of smokers as non-smokers is a much more important cause of bias than is the same rate of the reverse misclassification, of non-smokers as smokers. Since such reverse misclassification is also implausible, adult women having little reason to claim erroneously to be smokers, the major reviews^6–8 have all ignored its minor effects.

Given that in the Japanese spousal study …