Menstrual variability and the measurement of menstrual synchrony.

While the timing of menstrual cycles must be modifiable in order for the phenomenon of menstrual synchrony to occur, cycle variability and the occurrence of irregularly long or short cycles complicate the measurement of the process of synchronization over time and of determining whether synchrony exists for women who have lived together for a long period of time. The paper examines how each of five different ways of measuring menstrual synchrony (utilized in the literature) deals with menstrual variability and irregular cycles. On the basis of this analysis, the mathematical concept of "the limit of a sequence,' and analysis of data according to these five different approaches, we recommend the approach in which women record their onset dates after they have been together for a prolonged period of time. The observed onset differences between the women are then compared to expected onset differences, based on the assumption of random onset occurrence. A significant influence of irregular cycle lengths on synchrony was not found in this data set and in the other studies which examined this issue. Research to date has focused on a relatively narrow aspect of menstrual regularity and synchrony and has failed to emphasize the coupled pattern of events over time as well as to analyze menstrual synchrony as a cyclic system.