Studying a small number of cases in depth and comparing them to explain why outcomes differ.
Key thinkers: Charles Ragin, Robert Yin, Robert Stake
The longer story
Comparative case analysis examines similarities and differences across cases to explain why things happen as they do. Mill's methods—agreement, difference, concomitant variation—structure causal inference from comparison. Ragin's qualitative comparative analysis (QCA) uses Boolean logic for medium-N studies. The comparison reveals what matters: if outcomes differ when conditions differ, conditions might be causes. Case selection is critical—you need variation on what you're trying to explain.