Porter — The Growth of St. Louis Children. 289 
The mean or average of the observations at any age in the 
period of growth is typical of the child at that age, anda com- 
parison of the means at different ages will reveal the law of 
growth of the type. Again, the mean of the observations at any 
deviation from the mean of the whole number, for example 
the height at a deviation of + d from the mean, or, if Galton’s 
method is employed, the height at any percentile grade, is the 
type of those who stand at a certain degree of deviation from 
the type of the whole number. Thus types of tall and short, 
light and heavy children are secured. The types of the same 
degree of. deviation from the mean at all ages are as compar- 
able as the type of the whole number of observations, and 
reveal the growth of the typically tall and short, light and 
heavy children; but the comparison is less secure the greater 
the deviation from the mean, for the probable error is 
inversely as the square of the number of observations, and 
the number of observations rapidly diminishes on either side 
of the mean. 
The methods described in this chapter have been employed 
in the present investigation. For every entire series here 
presented, the mean and the average, the probable deviation 
and the probable error, and the 5, 10, 20, 25, 30, 40, 50, 60, 
70, 75, 80, 90 and 95 percentile grades have been calcu- 
lated. The 25 and 75 percentile grades were obtained by 
dividing by 2 the sums of the 20 and 30, and the 70 and 80 
percentile grades respectively. 
