If used perfectly every cycle, including abstaining during the fertile phase, the FAM rules have a failure rate of approximately 2% per year. This is considered lower than any barrier method, except the condom, which is also 2%. But unlike other methods of contraception, this method is the most unforgiving, since, if you make a mistake, by definition, it will occur during your most fertile time. So in actual use, failure rates may be higher, depending on the motivation of the couple to adhere to the rules.