TABLE 6 Exposure group Estimqted Mean and Standard Deviation for Age at Onset of Gray Hair ~ (by Sex and Exposure Group) Male , Mean Standard deviation Mean Standard deviation 1 45.5 10.6 50.0 9.5 2 46.4 6.2 51.0 6.0 3 48.8 8.0 49.5 9.0 4 47.0 11.1 50.5 7.0 the female groups suggests that hair plucking was the cause. Analysis of simple incidence of gray hair in the population discloses a sigmoid curve characteristic of a normally distributed variable. By meansof probit analysis,!° the mean and standard devi- ation was estimated for age of the population at onset of gray hair (table 6). It can be seen that women begin to gray somewhat later than men. This difference might be related to the common practice of hair dyeing among Japanese women. However, the hair roots were closely inspected during data collection, so that this probably represents a true sex difference. Comparison of the means for the 4 exposure groups was generally negative, except that the means for men differed by a suggestive amount (P was TABLE 7 between .05 and .10). This was largely the result of the high mean age of onset of gray hair in the distally exposed group 3. Comparison of the heavily exposed group | with the extrinsic control group 4 showed no significant difference in mean age at onset of gray hair (P = .2). Regression analysis of the gradations of hair graying was undertaken, with the ratings 0 to 3+ being replaced by the numbers 0, 1, 2, and 3. The fit was reasonably satisfactory for most age groups, and the slopes of the regression lines did not differ significantly. The slopes and the corresponding correlation coefficients are shown in table 7. Although the slopes of regression are similar in men and women, the age for womenis shifted to the right. This again indicates that Japanese women become Slopes of Regression Lines and Linear Correlation Coefficients for Arbitrary Graying Score in Relation to Age (by Sex and Exposure Group) Slopes of regression lines 34 Female Correlation coefficients Group Male Female Male Female 1 .235 .284 138 720 2 273 317 785 .678 3 .290 2715 -788 .735 4 .270 240 719 689 GERIATRICS, JANUARY 1961