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.
Edward R. Dewey
Volume IV of the Cycles Classic Library Collection contains 1380 reports of cycles period determinations by scientists, doctors, economists and cycles researchers. In these reports there is a tendency for certain periods of cycles to be reported more commonly. These common periods include the underlined periods above and some other periods such as 9.6 years, found in the 9.6 year cycle of lynx abundance and 3.39 years (40.68 months), found in the US stock market, which Dewey says is the most commonly found period. Dewey further said that: