What effect sizes mean in practice
Effect size measures how much improvement a therapy produces compared to no treatment or placebo. A small effect size (0.2) means a modest difference; medium (0.5) indicates clinically meaningful change; large (0.8+) shows substantial symptom reduction. These numbers come from randomized controlled trials comparing therapy outcomes before and after treatment.
Major psychotherapies tend to cluster in the medium to large range for depression and anxiety. CBT and IPT consistently show effect sizes around 0.9 to 1.2 for major depression, while psychodynamic and acceptance-based therapies show similar magnitudes. The variation within each modality often exceeds variation between them, suggesting therapist skill and client fit matter as much as theoretical approach.
Comparing modalities and conditions
Different therapies excel at different conditions. CBT dominates for specific phobias and panic, with effect sizes often exceeding 1.5. For PTSD, trauma-focused CBT and EMDR both show strong evidence. Psychodynamic therapy shows larger effect sizes for personality pathology and chronic relational patterns than for acute anxiety, reflecting its different theory of change.
All established therapies significantly outperform waitlist controls, but the choice between them often depends on client preference, availability, and the specific problem. Combined treatment (therapy plus medication) sometimes exceeds either alone, though not uniformly. The practical implication is that evidence-based therapy is more powerful than its modality label.