Case reports document and describe clinical observations of an individual patient or very small groups of patients: often, one patient is described, but sometimes clinical observations of two or three patients are described.

Case reports differ from case series, which involves a small collection of patients (often four or more) who share a common clinical feature, such as a rare condition or response to a novel therapy.^[1]

Though the terms “case study” and “case report” have been used interchangeably in the past, their difference lies in the approach. Case reports contain a detailed description of a single patient’s unique clinical findings (diagnosis, treatment, and outcome). Case studies, on the other hand, tend to involve a much deeper qualitative exploration into the case or cases involved.^[2]

The lack of a control group plus the descriptive approach of all three study types means that they can describe observations in detail, but they cannot establish causation. Even in instances where causation may seem likely, the small number of patients involved means these reports are not suitable for application to a broader population. However, each of them is valuable to help clinicians learn from rare or unusual instances in practice.

In short, case studies are a qualitatively deeper cousin of the case report, while case series are a collection of more closely related case report siblings.