<prompt> If 100 woman experience 20 accidental pregnancies after using a contraceptive method for 2 years each, Pearl index:0.1 per 100 women years100 per 1000 woman years10 accidental pregnancies per 1000 woman years using contraceptives10 accidental pregnancies per 100 woman years using contraceptives  <response>: Pearl index is defined as the number of failures of contraceptioni.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 SaheliCHHAYA 1.83 - 2.84 Sterilization 0.1 