404 Plant-Breeding 



population, hence it is only another method of expressing type. 

 It is found by multiplying the mid-value of each class (V) by 

 the number of individuals in that class (/), then summing the 

 products and dividing this sum by the total number of individ- 

 uals. The formula for this operation is L. 1 



n 



Thus : 



We would get exactly the same result if we arranged the bean 

 plants, in order of size, in. a single line, placing them end to end, 

 and then divided the total length of this line by 500, the number 

 of individuals in it. 



Average deviation. One way of expressing variability is to 

 find out by how much, on the average, any individual in the 

 population deviates from the mean, the constant thus secured 

 being termed the average deviation. This is ascertained as 

 follows : the amount by which each class differs from the mean, 

 or in other words, the deviation from the mean (designated by 

 D) is multiplied by the frequency of the corresponding class, 

 and then the sum of these products is divided by the total 



1 The Greek letter capital "sigma" () indicates that the sum of 

 a series of values is to be taken. 



The total number of individuals measured is designated by n. 



