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A case study is an in-depth investigation of a single individual, group, or event using multiple data sources (interviews, observations, documents, tests). Case studies are invaluable for studying rare phenomena, generating hypotheses, and revealing mechanisms that surveys miss. Famous examples include Phineas Gage (personality and the frontal lobe) and H.M. (memory and the hippocampus). Their major limitation is that findings from one case may not generalize to others.
Read a classic psychology case study (e.g., H.M. or Genie) and identify what questions only a case study could have answered and what questions it left unanswerable.
You already know the family of descriptive research methods — surveys, naturalistic observation, archival research — all designed to characterize phenomena across samples. The case study is a sharp departure from this logic. Instead of measuring many people shallowly, it investigates one person, one group, or one event exhaustively. The trade-off is explicit: you gain depth and mechanism at the cost of breadth and generalizability. Understanding when that trade is worth making is the central practical skill.
Case studies earn their place by answering questions that no other method can. Some phenomena are so rare that a sample of N=1 is the only data available. Phineas Gage, the railroad worker whose personality changed dramatically after an iron rod destroyed his frontal lobe in 1848, provided the first strong evidence that personality and social behavior are localized in prefrontal cortex — a finding no experimental design could have produced. H.M. (Henry Molaison), whose hippocampus was surgically removed to treat epilepsy, spent decades unable to form new long-term memories while retaining old ones and intact procedural learning, giving researchers an unparalleled window into memory system architecture. Neither finding could have been generated by an experiment, because you cannot ethically ablate brain structures in humans. The rarity and specificity of these cases is precisely what makes them scientifically powerful.
The methodological rigor of a case study lies in triangulation: using multiple independent data sources to build converging evidence for the same conclusions. A researcher studying an exceptional child prodigy should not rely only on IQ scores or only on teacher impressions — they should gather cognitive testing, behavioral observation, school records, interviews with parents, and video documentation. When diverse sources agree, confidence in the conclusions increases. When they conflict, the conflict itself becomes a finding worth explaining. This multi-method approach is what separates a scientific case study from an anecdote, which rests on a single observer's unverified account.
The generalizability limitation is real but often misunderstood. A single case cannot license statistical generalization to a population in the way a survey can. But it can license theoretical generalization: if the theory predicts that all organisms with intact hippocampi can form declarative memories, then one patient without a hippocampus who cannot form declarative memories is evidence against the universal claim. Logic, not statistics, is doing the work. This is why case studies are most powerful in combination with other methods: they generate hypotheses and reveal mechanisms that experiments then test at scale, or they stress-test theoretical claims by finding the exceptions.