Case series and exposure series: the role of studies without controls in providing information about the etiology of injury or disease.

Descriptions of exposure histories in persons with the same injury or illness ("case series"), and descriptions of outcomes in persons with the same exposure ("exposure series"), have the potential to contribute knowledge relevant to disease etiology in some special situations. The case series can be thought of as a primitive form of case-control study--one in which the controls are only implied. Similarly, the exposure series is a rudimentary type of cohort study. By keeping these analogies in mind, those who author or read studies without controls can assess the design or results for selection bias, confounding, or information bias. While studies without controls cannot supplant true case-control or cohort studies, they are relatively cheap and easy to perform, and there are circumstances in which they provide valuable information. Attention to design is needed to strengthen the results they provide.