Porter — The Growth of St. Louis Children. 341 
nearly the same, at both measurements, the probability is that 
the child is growing normally. This probability is greater 
than the general probability that a normal deviation is more 
likely to occur than an abnormal one, but its numerical value 
is wholly unknown. If, on the other hand, the two deviations 
are unequal, the probability is that the greater of them is 
abnormal, but the numerical value is here also unknown. 
How definitely the individualizing method could answer this 
question is difficult of conjecture, in the present lack of data, 
but certainly no answer whatever could be expected except 
after two measurements separated by a year’s interval, a year 
in which the unchecked cause of an abnormal deviation might 
inflict an irreparable damage on the organism. Such indefi- 
nite and fragmentary knowledge can never be the basis of a 
practical reform. Any solution of this problem which shall 
gain general acceptance must be easy to understand and easy 
to apply, and must give the probable degree of abnormality 
of any observed deviation. These conditions are, I believe, 
fulfilled by the following method. 
According to the theory of probabilities, the heights of a 
thousand individuals of the same class will arrange themselves 
as follows: — 
Between M+4d and M-+nd 3 individuals. 
és M+ 3d “ M-+-4a 18 “ 
i M-++2d = M-+-3d 67 fs 
ss M+ d “6 M-+2d 162 “ 
ac ce M+ d 250 sé 
“ce M ‘cc C= d 250 ce 
7 M— d - M—2d 162 i 
" M—2d - M—3d 67 ts 
ns M—3d = M—4d 18 ee 
3 M—4d “3 M—nd 3 as 
where M =the mean and d = the probable deviation. 
Let these be divided into seven groups: — 
I. All individuals between M-+nd and M-+3d 21 
Aime ss “ M+3¢@ ‘* M-+2d 67 
Wl. « ‘ & M424... 4+ - M+ 4d 162 
VI ss “e és M “ M+ d 500 
V 3 s “ M—d t M—2d 162 
IV 6é “ ‘ M—2d *¢ M—3d 67 
