Correctly classifying cohort studies versus case series in systematic reviews and implications for evidence-based decision making.

The strength of a systematic review hinges in large part on the quality of evidence it appraises. Observational studies, including cohort studies and case series, are positioned lower on the hierarchy of evidence, with cohort studies typically higher than case series. The often subtle differences between these study designs may lead to misclassification and can impact the strength of recommendations derived from such data. This manuscript offers an approach to delineate the differences between cohort studies and case series and provides clinical examples of these subtleties. By providing a simple approach for distinguishing cohort studies from case series, this manuscript seeks to assist researchers performing systematic reviews.