ioo BIOMETRY 



of the correlation between the filial mean and the 

 value of the parental class. 



Regression represents the extent to which the 

 average son is more like the mean of the general 

 population than his father is. Correlation, on the 

 other hand, indicates the amount by which the son is 

 more like his parent than he is to the average of the 

 general population. Thus, instead of being exactly 

 like their parents, children are said to show regression 

 towards the mean of the general population to which 

 both parents and children belong. 



In a case where the mean height of the fathers is 

 identical with the mean height of the sons examined, 

 and both are the same as the mean height of the general 

 population, the coefficient of regression is simply equal 

 to the reciprocal of the correlation coefficient between 

 fathers and sons. In actual practice this condition is 

 seldom realized, and it is then necessary to use a more 

 elaborate method in order to determine the value of 

 the regression coefficient. 



Professor Pearson has extended the idea of correla- 

 tion to the case of characters which are not capable of 

 exact quantitative measurement. This extension is 

 based upon the assumption that such characters follow 

 a normal law of distribution in their variation, just in 

 the same way as such a character as human stature was 

 found to do. There is considerable doubt as to how 

 far this assumption is justified, so that at the outset 

 we may feel disposed to attach less importance to the 

 actual values arrived at in this way than we should in 



