Abstract
The selection of cases is the starting point of case-control studies and obeys the fundamental and rather obvious principle that they should come from the same study population as the cases. Ascertaining exposure very often involves interviewing cases and controls about a variety of factors to which they may have been exposed, ranging from smoking habits through diet to medical history, depending on the purpose of the study. ‘Investigating other people's past experiences’ points out that whereas the prospective study observes events in their natural course from causes to possible effects, a case-control study observes the events in a reverse sequence, from effects to possible causes.
