Length and Variability of The Menstrual Cycle

To the Editor:—  It is becoming increasingly evident that the log-normal distribution is encountered more often in biological systems and processes than was previously thought.^1,2 A recent paper by Chiazze³ and others concerning the length and variability of the menstrual cycle affords a typical and intrinsically interesting example. The average menstrual cycle length, averaged over at least ten cycles, of 436 females aged between 15 and 19 abstracted from the data given by Chiazze et al, is detailed in the first three columns of the Table.These data were plotted on linear- and log-probability graph paper and it was found that the log-normal curve gives a better fit; notably for the latter part of the sample distribution. The sample distribution is in fact slightly positively skewed, more average cycle lengths occur at greater times than one would expect if the normal distribution operated.I have accordingly fitted a