410 Plant-Breeding 



of variability, allows of comparison only between similar organ- 

 isms or parts, between such characters as are measured in the 

 same denomination, as tubers with tubers, or height measured 

 in inches with height in inches. This is because it is not an 

 absolute, or abstract constant, but really represents a certain 

 number of feet, pounds, centimeters, or what not. And just as 

 we cannot compare 5 pounds with 5 inches mathematically, so 

 we cannot compare standard deviation in inches with that in 

 pounds. 



An undenominational abstract constant that will allow of com- 

 paring diverse variabilities, let us say, height with thickness, or 

 pounds with inches, is designated as the coefficient of variability. 

 It is found by dividing the standard deviation by the mean. 



The formula is X 100 and it is symbolized by (7. It is really 

 M 



only the standard deviation measured in terms of the mean. For 

 our beans the coefficient of variability for length is .3948 or since 

 it is usually read as percentage, 39.48 %. This constant is now 

 comparable with any other coefficient of variability for what- 

 ever character or in whatever denomination it may have been 

 measured. Thus we can compare the variability in the length 

 of beans in millimeters with their variability in breadth meas- 

 ured in millimeters or inches, or with height in men or sugar 

 content in beets, if we wish. 



Probable error. Probable error does not mean the amount 

 of error that an investigator is likely to make in his experiments 

 or measurements. It means that if he would measure another 

 random sample of a population similar in size and character to 

 the sample he had measured before, the chances are even that 

 the mean for the new sample would lie somewhere between the 

 limits denoted by the probable error. Thus, the mean as to 

 length of plants for our beans is 37.94 cm. with a probable error 

 of .4518. This means that the mean for the new population 

 would not be greater than 37.9400 + .4518 = 38.3918 cm., or 



