When used correctly, effectiveness of the method reaches nearly 100% and between 76% to 97% in typical use. The big gap in the typical use is created by referring to all the natural methods as one, while each one has a different efficacy. FAM has the highest efficacy of all, using the combination of 3 fertility signs.
From my experience as a teacher, and from my colleagues’ experiences, we noticed a very high efficacy of 98%, when using the method correctly. The 2% failure rate was based on the failure of other means of contraceptives during the fertile days.