Contraception efficacy is measured by

Two methods that have been used to measure contraceptive efficacy are the Pearl index and life-table analysis.Pearl index is defined as the number of " failures per 100 woman-years of exposure (HWY)"Failure rate per HWY = Total accidental pregnancies / Total months of exposure x 1200The Pearl index is usually based on a specific exposure (usually one year) and, therefore fails to accurately compare methods at various durations of exposure.The life-table analysis calculates a failure rate for each month of use. A cumulative failure rate can then compare methods for any specific length of exposure.Park 23e pg: 510