The calendar method is perceived to be less effective than other methods of family planning. A large existing data set was used to determine how well the fertile time is identified using the traditional calendar method formula and to determine if better formulas could be developed to identify the fertile time more accurately and require less abstinence. We compared the traditional formula with three alternatives, two of which were developed for this analysis. All three alternative formulas performed better than the traditional formula in identifying the presumed fertile time. The result of our analysis is a summary table which can be used to select the best rules for testing the effectiveness of the calendar method.
PIP: While it is difficult to assess the effectiveness of the calendar method of ovulation prediction, the method is perceived to be less effective than other means of family planning (FP). In order to derive a formula(s) that identifies the ovulation accurately and minimizes the number of days of abstinence required, data from the World Health Organization's trial of the Ovulation Method of Natural FP (in which examination of cervical mucus was used to determine ovulation) were analyzed. This analysis used cycle length and peak fertile day to develop and compare the traditional calendar method formula (which calls for abstinence from day 10 to 18, inclusively) to a blanket formula (which calls for abstinence from day 9 to 19, inclusively), a midcycle formula (which calls for abstinence from day 10 to 18, inclusively, for a woman whose cycles vary from 26 to 30 days), and an average midcycle formula (which calls for abstinence from day 8 to 20, if a woman's past six cycles were 28, 27, 26, 29, 30, and 28 days). For each formula, coverage at peak day was assessed. The average midcycle and blanket formulas predicted the fertile phase most accurately, and the traditional formula performed the least favorably (since peak day increase about one half-day instead of one-to-one as cycle length increases). Any introduction of a calendar method of FP should be accompanied by basic education about how the method works.