A study reports findings on Patient 1, Patient 2 and Patient 3, describing each in detail with no comparison group. What type of study is this?

Correct answer: D. Case series

• A. A CASE REPORT describes a SINGLE patient. Three patients makes this a series, not a report.
• B. An RCT requires random allocation to intervention and control arms. Neither element is present.
• C. A cohort study requires defined exposed and unexposed groups followed over time to compare outcome rates. There is no comparison group here.
• D. CORRECT. A CASE SERIES describes a group of patients — conventionally more than one, typically 3 or more — who share a condition or intervention, reported descriptively with NO control or comparison group. It sits low on the evidence hierarchy because without a comparison group it cannot support causal inference, but it is valuable for describing rare conditions and generating hypotheses.

Note: Evidence hierarchy from the bottom: expert opinion → case report → case series → cross-sectional → case-control → cohort → RCT → systematic review and meta-analysis.