A case series (also known as a clinical series) is a medical research descriptive study that tracks patients with a known exposure given similar treatment or examines their medical records for exposure and outcome. It can be retrospective or prospective and usually involves a smaller number of patients than more powerfulcase-control studies or randomized controlled trials. Case series may be consecutive or non-consecutive, depending on whether all cases presenting to the reporting authors over a period were included, or only a selection.
Case series may be confounded by selection bias, which limits statements on the causality of correlations observed; for example, physicians who look at patients with a certain illness and a suspected linked exposure will have a selection bias in that they have drawn their patients from a narrow selection (namely their hospital or clinic). Internal validity of case series studies is usually very low, due to the lack of a comparator group exposed to the same array of intervening variables. For example, the effects seen may be wholly or partly due to intervening effects such as the placebo effect, Hawthorne effect, Rosenthal effect, time effects, practice effects or the natural history effect. Calculating the difference in effects between two treatment groups assumed to be exposed to a very similar array of such intervening effects allows the effects of these intervening variables to cancel out. Hence only the presence of a comparator group, which is not a feature of case-series studies, will allow a valid estimate of the true treatment effect.