. "In the following explanation, the coefficient of variation of the thick-and-thin recurring length is calculated from data measured on 50 samples arbitrarily selected according to the following formula: ##EQU1## wherein li is the i-th value of the thick-and-thin recurring length in the 50 samples arbitrarily chosen, and L is an average value of the thick-and-thin recurring length of the above 50 sa" . . .