Most sources of error due to confounding and bias are more common in retrospective studies than in prospective studies.
For this reason, retrospective investigations are often criticised.
Case–control studies are often used to identify factors that may contribute to a medical condition by comparing subjects who have that condition/disease (the "cases") with patients who do not have the condition/disease but are otherwise similar (the "controls").
They require fewer resources but provide less evidence for causal inference than a randomized controlled trial.
Increasing the number of controls above the number of cases, up to a ratio of about 4 to 1, may be a cost-effective way to improve the study.
A prospective study watches for outcomes, such as the development of a disease, during the study period and relates this to other factors such as suspected risk or protection factor(s).
The case–control study is frequently contrasted with cohort studies, wherein exposed and unexposed subjects are observed until they develop an outcome of interest.
Controls can carry the same disease as the experimental group, but of another grade/severity, therefore being different from the outcome of interest.
The study usually involves taking a cohort of subjects and watching them over a long period.
The outcome of interest should be common; otherwise, the number of outcomes observed will be too small to be statistically meaningful (indistinguishable from those that may have arisen by chance).