ITT.—ASSOCIATION. ) 
tion appears to have been used: the data refer to natives only, 
and the numbers of observations are sufficiently large to reduce 
“fluctuations of sampling” within reasonably narrow limits. 
The proportions not attacked are as follows :— 
Proportion not Attacked. 
Not Inoculated. Inoculated. Difference. 
0-8776 0-9892 0-1116 
07687 09796 02109 
09698 09953 0-0255 
0-9901 0-9988 0-0087 
09935 0-9990 00055 
09941 0-9990 0-0049 
In cach case inoculation and exemption from attack are positively 
associated, but it will be seen that the several proportions, and 
the differences between them, vary considerably. Evidently in 
a very mild epidemic this difference can only be small, and the 
question arises how far the data for the separate epidemics can 
be said to be consistent in their indication of the “efficiency ” 
of the inoculation. This is not a simple question to answer: 
the more advanced student is referred to the discussion in the 
original. 
11. The values that the four second-order frequencies take in 
the case of independence, viz.— 
(A)B) (a)(B) (4)(B) (a)(B) 
ERAS REY. Bae ik 
are of such great theoretical importance, and of so much use 
as reference-values for comparing with the actual values of 
the frequencies (4.8) (aB) (48) and (aB), that it is often desir- 
able to employ single symbols to denote them. We shall use 
the symbols— 
4)(B 
(ap), =E (op), -@B) 
a)(B 4 
(B= AB) yp), DB) 
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