Cycle Length Variability


(If you are not familiar with the concept of standard deviation, please go to http://www.robertniles.com/stats/stdev.shtml for an easy-to-understand explanation.  Below “SD” is an abbreviation for standard deviation.)

There is disagreement as to how much cycle length variability is present among women participating in menstrual synchrony studies.  Is the length of most women's menstrual cycles about the same, or are there "large differences in cycle length?"

Weller and Weller describe how Schank used computers to "simulate a range of variation from no variation to just below (SD=5.3) greatest cycle variation we reported (i.e., SD=5.5, Weller et al., 1999)."  Weller and Weller explain that Schank then calculated that "a couple with different cycle lengths, 27 and 35 days, would have an expected cycle length [difference] of 7.307 and not 7.75 days as we calculated.”

Weller and Weller go on to state that "if [Schank's] calculation is correct, we would agree with him that our procedure may increase the chances to find synchrony in a sample containing a number of couples with large differences in cycle length."

But, after having performed "additional analyses on the Bedouin study's** family data," Weller and Weller found "a mean intra-family cycle length of 30.26 days with an SD of 1.52 for the first month, and 31.57 (SD=1.93) for the second month.  These SD values are in the low range of the values used in Schank's simulations (0-5.3).  Schank's arguments are mostly based on assumptions of much greater degrees of variability (e.g., SD=5.3, p.9).”

Yet, Schank questions the "mean cycle length of 30.26 days (SD=1.52) for the first month.”  Schank asks:  "This standard deviation is indeed much lower than they have typically reported, but was it computed correctly?  The mean for the first month reported by Weller and Weller (1997a) was 30.97 days for the same data (as extrapolated from table 1, p. 147, Weller and Weller, 1997a).  If the first month's mean cycle length was 30.26 with an expected difference of 7.57, why do all the expected scores range from 7.7 to 7.6 in their original publication (Weller and Weller 1997a)?  If the expected differences are not what they reported in Weller and Weller (1997a), they should be corrected.”

**  A key study in the menstrual synchrony literature.

Let’s take a closer look at variability in menstrual cycle length.


X Variability in Menstrual Cycle Length

X Menstrual Synchrony Index

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