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In 1946, the physician and statistician Joseph Berkson (1899–1982) pointed out that two diseases that are independent in the general population may become ‘spuriously associated’ in hospital-based case-control studies.1 This spurious association was later referred to, often in lively debates,2–14 as Berkson’s fallacy, Berkson’s paradox or Berkson’s bias. Some authors restricted the interpretation of Berkson’s fallacy to disease-disease associations,2,5,7,8 whereas others thought that the fallacy would also apply to exposure-disease associations in hospital-based case-control studies.10–15 In this article we use directed acyclic graphs (DAGs) to describe the structure of Berkson’s fallacy, first for disease-disease associations and then for exposure-disease associations. This permits us to understand the contentious debates and strongly differing opinions about Berkson’s fallacy, and has practical implications for study design and interpretation (see Box 1). Box 1. Practical implications of Berkson’s fallacy The fallacy that became eponymous for Berkson caused controversy from its initial formulation onwards. Some held that it biased all case-control studies in hospitals; others maintained that it was only pertinent for associations between prevalent diseases, and did not exist for exposure-disease associations. The DAG analyses in this paper show that Berkson’s fallacy can exist when studying exposure-disease associations, be it only in an ‘indirect’ form and in exceptional circumstances: when in a hospital-based case-control study persons with a prevalent diagnosis are enrolled who were hospitalized for another disease that is associated with this exposure. The DAG of the problem that Berkson originally described has the same structure of all biases due to conditioning on a collider, but cannot be endowed with a causal interpretation since it was formulated and worked out as a problem of the association of prevalent diseases. When using incident cases in hospital-based case-control studies, Berkson’s fallacy becomes highly unlikely for exposure-disease associations, unless there are many people who have developed two different new diseases more or less at the same time and are hospitalized for the other disease, i.e. the disease that is not the subject of the case-control study. When incident cases in a case-control study consist only of people who have been hospitalized for that disease, Berkson’s fallacy is not possible. It is likely that Berkson’s fallacy has had very limited, if any, impact on the findings of epidemiological studies.

Objectives To describe the association of diet and socioeconomic position and to assess whether two different indicators, education and occupation, independently contribute in determining diet. Methods A community-based random sample of men and women residents of Geneva canton, aged 35 to 74, participated in a survey of cardiovascular risk factors conducted annually since 1993. Lifetime occupational and educational history and a semi-quantitative food frequency questionnaire were obtained from 2929 men and 2767 women. Results Subjects from lower education and/or occupation consumed less fish and vegetables but more fried foods, pasta and potatoes, table sugar and beer. Iron, calcium, vitamin A and vitamin D intake were lower in the lower educational and occupational groups. Both indicators significantly contributed to determining a less healthy dietary pattern for those from low social class. The effects of education and occupation on dietary habits were usually additive and synergistic for some food groups. Conclusion Assessing both education and occupation, improves the description of social class inequalities in dietary habits, as they act, most of the time, as independent factors.

Epidemiological studies have consistently demonstrated that either occupational or recreational physical activity is protective against colon cancer. However, it is unclear whether recreational activity is similarly protective among those who engage in high or low occupational activity. We therefore compared 440 male cases of colorectal cancer with 1164 male hospital patients. Occupational activity was defined according to job title, while recreational activity was assessed by questionnaire for three different periods of life. Occupational activity was protective with respect to colorectal cancer irrespective of whether one engaged in recreational activity at any different period of life. In contrast, recreational activity, performed at 20-44 years of age appeared to decrease colon cancer risk by 10-25% irrespective of the intensity of job activity. The present results suggest that, although we observed a larger effect with occupational activity than with recreational activity, middle-aged men may reduce their risk of colorectal cancer if they exercise when they are not working. These findings need to be confirmed in other populations.

In November 1984, because of an accident at a chemical plant, the population of a large area in the town of Geneva, Switzerland, was exposed to toxic bromine gas during several hours at concentrations above the short-term exposure limit. We describe the development of the disaster, the reactions of the fire brigade, the police and the first aid services as well as the breakdown of communications that occurred. On-the-spot epidemiological assessment of the exposed area and of the characteristics of the exposure syndrome was combined with the emergency care of 91 self-referred patients that were seen at the University Hospital. Follow-up was obtained one month later. The clinical course was moderate (eyes and upper airways irritation) and self-limiting in all cases. Immediate definition of the exposed population permitted effective follow-up. Such early evaluation could prove to be of crucial importance for managing more serious accidents of a similar nature. Language: en