Menstrual cycle shortest and longest cycle lengths is considered very irregular. The average menstrual cycle lasts 28 days. The variability of menstrual cycle lengths is highest for women under 25 years of age and is lowest, that is, most regular, for ages 25 to 39. Subsequently, the variability increases slightly for women aged 40 to 44. The luteal phase of the menstrual cycle is about the same length in most individuals (mean 14.13 days, standard deviation 1.41 days) whereas the follicular phase tends to show much more variability (log-normally distributed with 95% of individuals having follicular phases between 10.3 and 16.3 days).
Menstrual synchrony and Schank followed up on this point by using computer simulations to estimate the average onset difference that would occur by among women with variable cycle lengths and a mean cycle length of 29.5 days reported by McClintock. They reported that the average onset difference by chance among women with cycle characteristics reported by McClintock was about 5 days. They also calculated the expected onset difference at the beginning of the study using McClintock's method for calculating initial cycle onset differences. They reported that the initial cycle onset difference for the groups of women using McClintock's method was about 6.5