Menstrual synchrony and cycle variability: A reply to Schank (2000)

Menstrual synchrony has been measured by a change over time, to capture the "process" of synchronization or by examining a sample of women who have been together for a number of months ("state" synchrony). We have proposed and used a method for calculating "state" synchrony. On the basis of the results of a computer simulation, Schank (Schank, J.C., 2000. Menstrual-cycle variability and measurement: further cause for doubt. Psychoneuroendocrinology 25, 837-847) argues that our method is biased towards finding synchrony and that the degree of error is an increasing function of cycle variability. We show that Schank's model is based on unfounded assumptions. Contrary to Schank's assertion, on which much of the critique is based, there is no evidence that cycle variability is actually large in samples with "state" synchrony. Our reports are valid; and our method is supported both by convergent validity and by the use of statistical "random control" groups.