<prompt> In Pearl index formula, Pregnancy rate per HWY:Total accidental pregnancies x 1200 Number of women observed x Total months of use of contraceptiveTotal months of exposure to unintended pregnancy X 1200 Accidental pregnanciesTotal accidental pregnancies x 100 Number of women Total months of exposure to unintended pregnancyTotal accidental pregnancies Total months of exposure to unintended pregnancy  <response>: The use-effectiveness of contraceptive method is expressed as Pearl rate life table technique. Pearl rate or pearl index was introduced by Raymond Pearl in 1932 The rate is expressed as pregnancy rate per 100 women years HWY calculated as Pregnancy rate per HWY Total accidental pregnancies x 1200 Number of women observed x Total months of use of contraceptive 1200 is the number of months in 100 years 