Case series (also known as clinical series)

Case series typically involve a much smaller number of patients than the more powerful case-control study or Randomised controlled trial (RCT). In this design patients with a known exposure to a treatment are tracked prospectively to identify new forms of disease or adverse effects. Case series can also be retrospective - here medical records are examined to identify a link between exposure and disease.

You can see how case series designs may be confounded by selection bias because physicians choose to study patients from their hospital or clinic who they suspect have an illness that may be linked to an exposure.

Case series can be consecutive or non-consecutive. The results of case series can generate hypotheses that are useful in designing further studies including RCTs. No causal inferences should be made from case series regarding the efficacy of an investigated treatment. Case series have no control group, so they have no statistical validity. The benefit of case series is that they are easy to understand and can be written up in a very short period of time.

A case series is simply a series of cases of disease which can raise awareness initially of a new disease or disease variant. Acquired immunodeficiency disease syndrome (AIDS) and bovine spongiform encephalopathy (BSE), were initially described from the excess occurrence of cases of related rare diseases, which eventually led to new disease definitions.