The Standard Days Method® is a fertility awareness-based method of family planning. Users avoid unprotected intercourse during cycle day 8 through cycle day 19. The method is most effective for women with cycles that usually range 26-32 days. It was developed by applying various formulae (i.e. various numbers of days and various sets of days) to over 7500 menstrual cycles in an existing data set from the World Health Organization, to determine which formula would provide the optimal balance between he length of the identified fertile period and the efficacy in avoiding unplanned pregnancy.