New methods of mathematical analysis for studies of intrauterine contraception

The authors present two new procedures of mathematical analysis to evaluate the incidents which arise during the use of any contraceptive method. These two new approaches, which the authors call the Probability Index (PI) and the Weighted Average Index (WAI), are compared to those by Pearl and Tietze. Their conclusions are that the PI, although the best estimator of real probability, is not as functional as the WAI. The latter is not only much easier to calculate than PI, but also presents a greater correlation, prediction accuracy and discriminant power than the Pearl and Tietze indexes.