The two groups are fundamentally different in principle: 1. Historical cohort study: According to the facts exposed by researchers in the past at the beginning of the study, and according to the exposed historical data, the research population was divided into exposed group and non-exposed group. At this time, the incidence rate or some kind of health outcome of exposed group and non-exposed group were collected, and the differences between the two groups were compared by statistical method, so as to calculate the incidence rate. 2. Case-control study: to analyze the risk factors of a certain disease, we should start with the existing patients, take the patients diagnosed with a certain disease as cases, and take comparable individuals who do not have the disease as controls; Through inquiry, laboratory examination or medical history review, the previous exposure history of risk factors was collected, and whether there was a statistical difference in the exposure ratio of each factor between the two groups was compared. The sample size of case-control study that can't calculate the incidence rate is much smaller than that of historical cohort, so it can be used for the study of rare diseases. See EpiMan| Public Health Website for the answer. & gt