A prospective study monitors the outcome within a study group and relates it to suspected risk or preventive factors. A retrospective study relates the outcome to risk and preventive factors present prior to the start of the study. Prospective studies involve larger study groups than retrospective studies.
The regular testing and analysis involved with prospective studies makes them more reliable for identifying risk factors. Retrospective studies have higher instances of bias and confounding variables than prospective studies. Relative risk calculation is best determined by retrospective studies. Retrospective studies frequently provide the underlying methodology for observational and case-control studies. The Lane and Claypon's 1926 study of risk factors for breast cancer was retrospective.