In a prospective study, the investigators conduct an investigation on a group of subjects and analyze the outcomes. In a retrospective study, investigators select groups of subjects who have already been subject to an intervention and analyze how the intervention relates to the outcomes. Large studies are stronger than small studies. The sample size of a study should be large enough to have an acceptable probability of detecting a difference in outcomes, if such a difference truly exists, between the experimental and control groups attributable to the intervention being evaluated. Although larger studies increase the statistical power of the evaluation, there is a point beyond which there are diminishing returns and studies may become unnecessarily costly and inefficient.
Contemporaneous controls are stronger than historical ones.
A contemporaneous control group exists when the results of an intervention group and a control group are compared over the same time period. An historical control group exists when the results of an intervention group are compared with the results of a control group observed at some previous time.