The Measurement of Variation 49 



nor on 



Average deviation = = 3.24 inches. 



Standard deviation, (<r) = / 485L31 = 4.1+ i n . 



Coefficient of variability. The average deviation or 

 standard deviation as outlined above is always determined 

 in the denomination of the unit in which the plant is 

 measured ; if it is height of plant in inches, the deviation 

 will be in inches and so forth. This prohibits the careful 

 comparison of the deviations of different plants or parts 

 of a plant because some deviations may be in pounds 

 or others in inches, and hence they will not be directly 

 comparable. 



It is desirable, therefore, to have an abstract expression 

 so that the relative amount of variability of one class of 

 organs may be directly compared with the variability of 

 another. This is called the coefficient of variability. It is 

 found by dividing the standard deviation by the mean. 

 Thus an abstract number is found which expresses the 

 variability. In our case the standard deviation = 4.1 

 inches and the mean =15.5 inches, so that 



.264 = 26.4 % = the coefficient of variability. 



15.5 



If the coefficient of variability of the weight of the plants 

 had to be determined and was found to be, say, .384, it 

 would follow at once that the height of the plant was 

 considerably more variable than the weight. 



The coefficient of variability may be expressed as 

 follows : 



