Porter — Relation of Growth to Probable Deviation. | 235 
Thus, in the following table, comparing the Relative Probable 
Deviation from the average height standing of boys with the 
square roots of the number of observations, the Probable 
Deviation should be much greater at ages 17 and 18, in which 
the number of observations is small, than at age 10 or 11, in 
in which the observations are much more numerous. A look 
at the figures shows that the Probable Deviation is very little 
Age at nearest Number of Square Relation of Probable 
Birthday. Observations, Root. Deviation to Average. 
6 709 26.63 3.1 % 
7 1850 43.01 3.2 
8 2223 47.15 3.3 
9 2205 46.95 3.0 
10 2087 45.68 3.1 
11 1819 42.63 3.2 
12 1653 40.67 3.2 
13 1268 35.62 3.5 
14 925 30.42 3.8 
15 490 22.14 4.1 
16 189 13.75 3.7 
17 78 8.85 3.1 
18 29 5.40 2.8 
influenced by variations in the number of observations, within 
the limits given here. The Probable Deviation may, therefore, 
without any error of importance, be considered as the Physi- 
ological Difference between the Individual and the Type. 
Not all observers have taken the Median Value as the Type. 
The arithmetic mean is frequently employed in Germany, 
Denmark and elsewhere. In a large series the difference — 
between the two is so small that either may be safely used. 
The maximum andthe mean Median minus Average values for 
the physical dimensions studied in this paper are as follows: — 
MEDIAN MINUS AVERAGE VALUE. 
Unit OF MaxIMuM. neh nl 
DIMENSION. Maasteeuex?. | —<————————_ | 
Boys. | Grrts.| Boys. | Girts. 
sieht. at EPPS is A 2 0.74 0.23 0.25 
Height Standing...| Centimetre-...-.-| 1.00 1.10 0.50 0.49 
Height Sitting.....| Centimetre....---| 0.94 0.99 0.44 0.67 
pan of Arms....--| Centimetre....--.; 1.35 1.38 0.53 0.59 
Girth of Chest.....| Centimetre ---+...; 0.84 0.71 0.44 0.46 
There can, therefore, be no objection to the use of the 
