THE STATISTICAL STUDY OF VARIATION 



53 



moment, and the summation of all. the product-moments divided by n 

 is the average product-moment. This measure of absolute correlation is 

 expressed by the formula 



Av. prod.-mom. = 



n 



No. of culms per plant 

 Gx = 4 

 2 345 



7 / 



f.d\ 



FIG. 26. Calculation of coefficient of correlation (r) for total yield of plant in grams and 

 number of culms per plant for Sixty Day oats grown at Ithaca, N. Y., in 1910. 



But we cannot compare such a number as derived for instance from 

 size of potatoes and starch content with size of beets and sugar content 

 without reducing them to a relative basis. Pearson suggested the product 

 of the two standard deviations as the best index of variability by which to 

 divide the average product-moment so as to reduce it to a relative basis. 



