160 Essay on the Interpretation of Milk Records. 



Raymond Pearl says/ " It appears to be generally the case 

 that fecundity, fertility, and closely allied characters are highly 

 variable. There would seem to be a tendency for the 

 coefficients measuring variation in these characters to be above 

 a value of 20." 



The amount of variation in milk yield of cows is therefore 

 in no way exceptional. According to the present data, it is 

 about equal to that found in swine with regard to the size of 

 litter and not much more than half that of man in respect to 

 size of family. 



It was next required to find whether the average, maximum, 

 and revised maximum really were closely connected with the 

 total lactation yield which the owner might expect to obtain 

 from his cow under ordinary conditions. The similarity of the 

 variation curves does not, of course, of itself prove any 

 connection. 



The correlation between totals and the three suggested 

 systems was therefore determined for the 1,233 normal lacta- 

 tions, with the following results : 



Correlation — 1,233 "Normal Lactations." 



Since the limiting restrictions of these so-called normal 

 lactations are by no means narrow (i.e. they include cows whose 

 lactations vary from thirty-five to forty-five weeks inclusive, 

 that were served any time up to sixteenth week after calving, 

 and that calved at all times of the year) these coefficients of 

 correlation seem very satisfactory, and high enough to justify 

 the use of any of the above figures as determinants of a " normal " 

 cow's yield. 



^ From Raymond Pearl. Study of Egg Production. U.S. Bureau Animal 

 Industry, Bull. 110. 



1 7- = 2 Dx D,, - 1 . 



n (Tx <Ty 



2 P.E,. = ± 0-6745 (l-r-Q 



