Pearl index is defined as the number of "failures of contraception(i.e, accidental pregnancies) per 100 woman-years of exposure (HWY)" Failure rate per HMY = Total accidental pregnancies X 1200 ----------------------------------- Total months of exposure Thus, on entering values given in question, we get 10 as answer. ** A minimum of 600 months of exposure is usually considered necessary before any firm conclusion can be reached regarding the contraceptive efficacy of a method Contraceptive Method Pearl Index (Per HWY) Calender method 24 Coitus interruptus 18 Male condoms 2-14 Female condoms 5-21 Diaphragm 12 Vaginal sponge Parous women Nulliarous women 20-40 9-20 IUCD 1- 5 OCP 0.1- 2.0 Centchroman (Saheli/CHHAYA) 1.83 - 2.84 Sterilization 0.1