X.—CORRELATION : ILLUSTRATIONS AND METHODS. 199 
ceeding to the second differences, t.e. by working out the successive 
differences of the differences in col. 3 and in col. 5 before corre- 
. 0 ED ry - - pz \ 
200, *%° g 
rc 
: : 
-. 8 
, 
oN - f 
§ 
- = v _ eo ta A vot 
Years t. 
Fig. 41.—Infantile and General Mortality in England and Wales, 1838-1904. 
lating. It may even be desirable to proceed to third, fourth or 
higher differences before correlating. 
" 
7 linet 
1. 30. 
L 
iv -— 
3 3” - 
rs - 
3 
a —_— tpi]. “ot 
1855 6. ( ‘ou io bv bo <7 £5 1900 05 ~ 
Fic. 42. —Marriage-rate and Foreign Trade, England and Wales, 1855-1604. 
15. Illustration v.—The two curves of fig. 42 show (1) the 
marriage-rate (persons married per 1000 of the population) for 
England and Wales ; (2) the values of exports and imports per 
head of the population of the United Kingdom for every year 
from 1855 to 1904. Inspection of the diagram suggests a similar 
relation to that of the last example, the one variable showing a