Calendar methods of fertility regulation: a rule of thumb

"[Many] illiterate women, particularly in the third world, find [it] difficult to apply usual calendar methods for the regulation of fertility. Some of them are even unable to make simple subtractions. In this paper we are therefore trying to evaluate the applicability and the efficiency of an extremely simple rule which entails only [the ability to count] a number of days, and always the same way." (SUMMARY IN ITA)