Length of the Observation Period as a Factor Affecting the Contraceptive Failure Rate

The contraceptive failure rate i.e. the number of pregnancies/100 years of contraceptive exposure is so sensitive to the length of the observation period that the factor of observation length should be a control in any comparative study of contraceptives. Sensitivity to observation length is greatest when contraceptive effectiveness is low and when members of the sample vary greatly in their individual efficiencies. It is seen that when mean efficiency of contraception is held constant the failure rate drops consistently as the observation period lengthens. The failure rate drops most dramatically during length of observation period as one moves from the most to the least efficient contraceptive methods. These results assume that postpartum amenorrhea has been controlled and that no couple has deliberately stopped contraception before the end of the observation period.