Measure contraceptive failure rate per 100 woman-years.
Pearl Index = (Pregnancies / Woman-months) × 1200. It expresses the number of pregnancies per 100 woman-years of contraceptive use. Lower values indicate more effective methods. Reference: combined pill ~0.3 (perfect use), condoms ~2–18, IUD ~0.1–0.8, no method ~85. The Pearl Index has limitations with long-term studies.