Jainer & Shivanandaswamy's comments about the problems of bias in case–control studies are well made. However, our study Reference Mason, Rimmer, Richman, Garg and Johnson1 was designed to avoid such problems by recruiting all patients with a likely diagnosis of schizophrenia in contact with general practitioners in a defined catchment area. There was no possibility of influencing the selection of individuals since they were all patients with a diagnosis of schizophrenia on a community mental health team's case-load.
The community mental health team concerned looked after an area of high socio-economic deprivation and the study included patients who had drifted down the social scale from more affluent rural areas where one would expect a lower prevalence of middle-ear disease. If there is any bias in this study it is likely to favour the null hypothesis rather than that suggested by Jainer & Shivanandaswamy.
In addition, perhaps the most striking finding in this study was the excess of left-sided middle-ear disease. In this case, the odds ratio of 4.15 meets the recommendation of Sackett et al Reference Sackett, Straus, Richardson, Rosenberg and Haynes2 that an odds ratio of greater than 4 should be used to establish an association in case–control studies.
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