Epidemiologists use population experience to learn about associations between environmental factors, lifestyles, food intake, treatments, poverty, genes, etc., and diseases. We do that by observing and analyzing what people do to themselves or what is being done to them related to the health problems they have. To be a student of the occurrence of diseases as a function of different exposures is to be a student of epidemiology. Since cause–effect relations unfold over time, both in terms of age and calendar time, time plays a crucial role in these analyses and disease occurrence is best studied from the start of exposures and in the time to follow, especially for exposures that change over time, as it can be done in a follow-up study. To obtain information on this population experience a more cost-effective strategy can sometimes be applied by sampling cases (rather than exposed and unexposed) and so-called controls.