Why is randomization not applicable in case-control studies?

In case-control studies, the rationale behind the choice is based on the fundamental structure of this type of research. In these studies, researchers start with participants who already have the outcome of interest (the cases) and compare them to participants who do not have the outcome (the controls). Since the main objective is to look back at prior exposures or risk factors that might correlate with the outcome, the participants’ statuses as either cases or controls are predetermined based on the occurrence of the outcome.

Due to this retrospective nature, randomization cannot be employed effectively. Randomization typically involves assigning participants to different groups before the outcome occurs, which is essential in experimental designs to ensure that the groups are comparable. However, in case-control studies, since the outcome has already taken place, you cannot randomly assign participants to either group after the fact. This structural limitation sets case-control studies apart from randomized controlled trials and other methodologies where randomization is feasible and beneficial for controlling biases.