The Pearl Pregnancy Index reexamined: still useful for clinical trials of contraceptives.

Data from 25 studies of intrauterine contraceptive devices show close agreement between the Pearl Pregnancy Index and a cumulative, 12 month, life-table pregnancy rate. Analysis of data from the early stages of these clinical trials shows the Pearl Index to be superior to the short-term life table in predicting the outcome of the study. The error introduced by using the Pearl Index as a single measure of contraceptive efficacy in clinical trials, under conditions in which those trials are often carried out, is probably very minor compared to the effect of situational factors that affect studies and to differences among various life-table methodologies. The real strength of the life-table analysis of contraceptive performance is not the single, summary, 12 month pregnancy rate it provides, but the complete picture of various reasons for termination over time, particularly when a large number of woman-months of experience has accrued.