Menstrual synchrony
Strassmann argued menstrual synchrony defined as menstruation overlap should be quite common. For example, the expected difference by chance between two women with 28-day cycles—which is approximately the average length of menstrual cycles of women at the age —is 7 days. Considering that the mean duration of menses is 5 days and the range is 2 to 7 days, the probability of menstruation overlap by chance should be high.
Calendar-based contraceptive methods
Developed by Georgetown University's Institute for Reproductive Health, the Standard Days Method has a simpler rule set and is more effective than the rhythm method. A product called CycleBeads was developed alongside the method to help the user keep track of estimated high and low fertility points during her menstrual cycle. The Standard Days Method may only be used by women whose cycles are usually between 26 and 32 days in length. In this system: