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