282 
On Theories of Association 
equal to twice the probable error ; where this open dot is seen, there twice the 
probable error fails to reach the true value of r, or the mean value of Q as the 
case may be. The failures of Q are seen to be three times as many as those of r t . 
Diagram XXII. Values of association Q for a typical table of ordinary statistical practice. 
10 
1-0 
* ■ »: • 
6 J 1 
1 0 
10 2-0 3-0 4 0 50 60 70 
Percentage of total in smallest quadrant. 
8 0 
9 0 
We may conclude justifiably that even as a coefficient of association r t is much 
more stable than Q for the usual type of distribution ; and we have already seen 
that this is so even for many marked cases of skewness. It is always an approxi- 
mation and often a very good approximation to the true correlation. On the 
other hand Q conveys no idea to the mind at all, except in as far as it is an 
approximation, usually very bad, to the same correlation. 
