112 



DOMESTICATED ANIMALS AND PLANTS 



Variability as to Length of Ear — Standard Deviation 



Mean = 8.514. 364.41 ^ 286 = 1.2742. 



V1T2742 = 1.13 — = standard deviation (cr). 



This Standard deviation is considered, therefore, as the uni 

 versal measure of variabiUty, and the student will do well to 

 work these values with original measurements until they come 

 to have a real meaning. After this has been done for a time 

 standard deviation will express as much about variability as does 

 the radius about a circle. 



Coefficient of variability. But one further step is necessary 

 in the mathematical study of variability. The mean length of 

 ear in this case was 8.514 inches, and its variability, that is, its 

 standard deviation, was i . 1 3 inches ; the mean weight of ear 

 was 8.807 ounces, and the standard deviation was 2.854 ounces. 



How now can we compare variability in inches with varia- 

 bility in ounces } In other words, how can we tell whether 

 this corn is more variable with respect to length than it is with 

 respect to weight, or vice versa } We cannot tell by direct 



1 The plus sign denotes that decimals are dropped. 



