Case reports are useful for identifying new or rare adverse effects but are subject to which bias?

• A. Selection bias
• B. Observer bias
• C. Reporting bias
• D. Publication bias

Answer: (C)

Case reports identify new or rare adverse effects by describing what was observed, but they rely on what the author chooses to report. This leads to reporting bias: selective disclosure of findings and outcomes, with a tendency to emphasize unusual or dramatic events while omitting other information. As a result, the reported association between a drug and an adverse effect can seem stronger or more common than it really is, and you can’t determine how often the effect occurs or prove causality from a single report. Other biases can influence case reports—observer bias can affect how symptoms are interpreted, and publication bias reflects a preference for publishing more dramatic or novel findings—but the bias most inherent to case reports themselves is reporting bias.