Variation in the Milk of Ayrshire Cows 59 



The two characters dealt with irf this study are (a) aver- 

 age milk yield per week in gallons, and (b) average fat per- 

 centage. The values for the former were obtained by dividing 

 the total yields as given in the Reports by the weeks in milk. 

 The fat percentage figures were taken directly from the Re- 

 ports. The ages were taken as centering at the mid-point of 

 each year. That is, for example, all cows recorded at 3 years 

 or more in age, but less than 4 years were put in the 3-year 

 class in the tables of the present paper. A 3-year old hence 

 is to be taken as including anything between 3 and 4 years. 



The Comparative Variability of Milk Production 



Milk production is essentially a physiological character. 

 It is a matter of some interest and significance to examine the 

 variability of the character in comparison with other physiolog- 

 ical characters, and also with some that are more strictly mor- 

 phological, as, for example, bone measurements. Such com- 

 parisons may be made through coefficients of variation. It 

 must, however, always be kept clearly in mind just what a 

 coefficient of variation is, and care must be taken to avoid 

 drawing too sweeping or even entirely unjustified conclusions 

 from comparison of these constants. What the coefficient of 

 variation measures is the percentage which the "scatter" or 

 variation exhibited by a distribution as measured by the stan- 

 dard deviation, is in the mean of the character varying. For 

 some purposes this percentage is meaningless. 



In Table 1 (page 62) are given coefficients of variation 

 for a number of characters for purposes of comparison with 

 milk yield. The coefficients are arranged in order of descend- 

 ing magnitude. 



This table brings out the well-known fact, which has been 

 discussed in some detail by Pearl, Gavin, and others, that, in 

 general, physiological characters exhibit high coefficients of 

 variation as compared with strictly morphological characters. 

 Characters which are intermediate in their quantitative deter- 

 mination, as, for example, the length of the egg in the domestic 

 fowl, give coefficients of variation intermediate in value. Pure- 

 ly physical characteristics which are usually regarded by physi- 

 cists and chemists as "constants," such as the specific gravity 

 of eggs, show very low coefficients of variation. 



