Development of new formulas to identify the fertile time of the menstrual cycle.

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.