226 
No. LXXIL—STATISTICS. 
The topics suitable to statistics are too numerous to specify, 
they include everything to which such phrases as “ usually,” 
“ seldom,” “ very often,” and the like are applicable, which vex 
the intelligent reader by their vagueness and make him 
impatient at the absence of more precise data. 
The principal things to be borne in mind in making 
statistical enquiries are :— 
1. That the groups with which they deal should be homo¬ 
geneous. Ex. It would be correct to inquire into the average 
height, and the frequency of different degrees of deviation 
from it, of greyhounds, and similarly as regards the speed of 
racehorses ; but it would be absurd to talk of these averages 
as regards dogs or horses generally, because there are many 
varieties of them differing greatly and irregularly in height 
and speed (see further remarks in par. 5). 
2. When the homogeneous groups are largely governed by 
the variation of a dominant influence, it is necessary to split 
them up into subdivisions, each referring to a short phase of 
the variation. Ex. It is correct to seek the average height 
of boys between 11 and 12, 12 and 13, and soon, but it would 
be absurd to seek that of boys generally. 
3. To select cases on a system wholly independent of the 
quality about which the inquiry is made. Ex. It would be 
correct to estimate the stature of the male adults of a nation 
by measuring individuals selected by lot; but it would be 
incorrect to take townsmen alone, and still worse those who 
lived in an unhealthy suburb of a town. It requires keen 
observation and much wariness to avoid errors due to a 
neglect of this caution, because phenomena that appear inde¬ 
pendent are often linked together in indirect and subtle ways. 
Ex. If we were to select the upper classes of Londoners 
according to the initial letter of their names, as printed in the 
Court Guide, and were to choose the letter Z for the purpose, 
