In a nested case-control study on breast cancer among airline cabin attendants, Kojo and colleagues¹ state this policy implication: “There is no need to take occupational factors into account in breast cancer prevention among cabin attendants”. With respect to breast cancer risk among cabin attendants, Kojo et al have not shown the absence of effect of an occupational exposure. There is a well known definition of a negative study, but the study of Kojo et al does not fit into that definition. A true negative study must be large and sensitive, and it must have accurate exposure data.² The study is lacking in these aspects. The material contains 27 cases. Information on exposure was collected retrospectively and therefore 8 cases (18%) and 24 non-cases (2%) were deceased. Thus proportionally more cases than non-cases were lost because of high mortality due to the disease under study. Furthermore, there was 52% participation rate in the questionnaire survey on exposure.³

In attempt to evaluate possible bias because of poor participation, Kojo et al calculate the odds ratio for breast cancer for all subjects in the cohort with …