IX.—CORRELATION. *3 
in fig. 32, nor coincident as in fig. 33, but standing at an acute 
angle with one another as ZR (means of rows) and CC (means of 
columns) in figs. 36-8. The complete problem of the statistician, 
like that of the physicist, is to find formule or equations which 
will suffice to describe approximately these curves. 
9. In the general case this may be a difficult problem, but, in 
the first place, it often suffices, as already pointed out, to know 
merely whether on an average high values of the one variable 
show any tendency to be associated with high or with low values 
of the other, a purpose which will be served very fairly by fitting a 
Fre. 33. 
straight line ; and further, in a large number of cases, it is found 
either (1) that the means of arrays lie very approximately round 
straight lines, or (2) that they lie so irregularly (possibly owing 
only to paucity of observations) that the real nature of the curve 
is not clearly indicated, and a straight line will do almost as well 
as any more elaborate curve. (Cf. figs. 36-38.) In such cases 
—and they are relatively more frequent than might be supposed 
—the fitting of straight lines to the means of arrays determines 
all the most important characters of the distribution. We might 
fit such lines by a simple graphical method, plotting the points 
representing means of arrays on a diagram like those of figures 
36-38, and “fitting ” lines to them, say, by means of a stretched 
black thread shifted about til] it appeared to run as near as 
1H