How do single-subject and group research approaches relate to one another in advancing psychological knowledge?

A researcher studying a new anxiety-reduction technique first demonstrates its effectiveness with a detailed, repeated-measures design on three individual clients, then conducts a large-scale experiment with 200 randomly assigned participants to test whether the effect generalizes. This approach of combining findings from individual-level and large-sample studies strengthens the overall evidence because the detailed individual data reveal how and why the technique works, while the large-sample data reveal whether the effect holds across a broad population.