Random Groups Method


Another point of statistical contention is the "random groups method."  Weller and Weller, the researchers who have done the most recent menstrual synchrony studies, construct a "random control group" in the following manner:


"...using a table of random numbers, the participants in the experimental group are randomly paired with each other.  The synchrony scores obtained from the real pairs are then compared with those from the statistically constructed random control group, by means of paired t tests."


Weller and Weller go on to state that:


"Remarkably, in every study, including McClintock's original study and a later study of estrous synchrony in rats, when such comparisons were made the experimental group was always significantly more synchronous than the random control group."


Schank replies as follows:


"The [random control group] procedure is invalid because the actual sample and constructed sample are not independent.  For example, if all subjects are randomly repaired, the mean cycle length and variance will be identical for the two groups."


Schank then goes to on critique a specific example, "Weller and Weller (1993) [applying] the random groups method to their dormitory samples":


"For the real data, they report N=108 pairs, but their "randomly constructed" control group, the constructed N=93 pairs.  Why the asymmetric sample size?  The mean synchrony score of real data was approximately 7.0 days and for the randomly constructed control group, 7.3 days...They reported p < 0.01 using the Mann-Whitney U-test.  However, 7.0 days was the expected onset difference.  Thus, they found a significant difference between two non-synchronous samples!  The random group method, far from being "The most serious criticism of Schank's contentions..." further illustrates the unreflective misuse of statistical methods in their research."


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