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The next type of epidemiologic study is the retrospective study. Based on the concept of the past. It is past, because we know the disease conditions of the subjects at the time of beginning of the study. In this type of study, we usually select both a group of diseased and normal healthy subjects, and try to find out what factors potentially cause people to get a disease. Usually, one interviews the subjects with regards to potential risk factors. Though outcomes is prone to a memory bias, retrospective study is cheaper, obtains results in a short period of time, and is the best method for studying rare diseases.