20.3.3 Case Series
A case series describes characteristics of a group of patients with a particular disease or patients who have undergone a particular procedure. A case series may also involve observation of larger units such as groups of hospitals or municipalities, as well as smaller units such as laboratory samples. Case series may be used to formulate a case definition of a disease or describe the experience of an individual or institution in treating a disease or performing a type of procedure. Case series should comprise consecutive patients or observations seen by the individual or institution to minimize selection bias. A case series is not used to test a hypothesis because there is no comparison group. (Occasionally comparisons are made with historical controls or published studies, but these comparisons are informal and should not include formal statistical analysis.) A report of a case series should include the rationale for publishing the population description and inclusion and exclusion criteria. Case series are subject to several types of biases, and therefore authors should be particularly careful about the kinds of conclusions that can be drawn from them.