Some principles of epidemiologic studies

The use of epidemiological evidence is frequently involved in clinical decision making. Usually epidemiological investigation seeks to determine the extent of the association between a suspected factor and the occurrence of a disease. When the data are collected retrospectively then rates of exposure to the factor are compared for a group of cases of the disease and a group of controls. If the study is prospective then incidence rates of the disease are compared for a group exposed to the factor and an unexposed group. Although the retrospective approach is often more feasible it is also more vulnerable to bias. The possible influence of bias and chance must be taken into consideration when interpreting the results of any study. Relative risk serves as a useful index for expressing the strength of the association between the factor and the disease.