Menstrual synchrony by chance when there is menstrual cycle variability. Yang and Schank argued that when there is cycle variability (i.e., either women have irregular cycles, have cycles of different frequencies, or both), most women will have the opportunity to experience synchrony even though it is a result of cycle variability and not a result of a mechanism such as the exchange of pheromones. For example, consider two women A and B. Suppose A has menstrual cycles that are 28 days long and B has cycles that are 30 days long. Suppose further that when A and B become close friends, B
Menstrual synchrony 7.5 days. The mean difference in cycle onset was calculated for the beginning, middle, and end of the study for the pairs and triples of women. Ziomkiewicz found no statistically significant difference from the 7.5 day expected difference at either the beginning, middle, or end of the study. She concluded that there was no evidence of menstrual synchrony. H. Clyde Wilson argued that at the start of any menstrual synchrony study, the minimum cycle onset difference must be calculated by using two onset differences from each woman in a group. For example, suppose two women have exactly 28-day cycles. The