Menstrual synchrony
Strassmann argued menstrual synchrony defined as menstruation overlap should be quite common. For example, the expected difference by chance between two women with 28-day cycles—which is approximately the average length of menstrual cycles of women at the age —is 7 days. Considering that the mean duration of menses is 5 days and the range is 2 to 7 days, the probability of menstruation overlap by chance should be high.
Menstrual synchrony
In addition to the study they conducted with lesbian couples, Weller and Weller conducted a number of other studies on menstrual synchrony during the 1990s. In most studies they reported finding menstrual synchrony, but in some studies they did not find synchrony. In a methodological review article in 1997, they refined their approach to measuring to better handle the problem of cycle variability. Specifically, they concluded that several menstrual cycles should be measured from each woman and that the longest average cycle length in a pair or group of women should be the basis for calculating the expected cycle onset difference. Thus, their research falls into the pre-1997 methodology and post-1997 methodology.