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The most common practice to analyze epidemiological follow-up studies is to analyze risk factors of new, i.e. incident, cases of disease. However, analysis of incidence assumes that diseases exist in true dichotomies, which is unlikely to be true. It has also recently been shown that in many typical situations it is very difficult to separate the association between risk factors of disease at baseline and during follow-up using analyses of incidence. Situation is especially problematic for diseases that have large misclassification and low incidence, like asthma. We suggest that reliance on analysis of incidence may be a major obstacle into discovering causes of such disease. Only with greater attention into how to define and how to analyze prospective studies are we likely to learn sufficiently of risk factors of such disease to finally arrive at means for their prevention.