The Variation of Milk Secretion with Age. 57 



This opinion has been shown to be erroneous by previous work 

 of this laboratory on the statistics of the 7 day records of Ameri- 

 can Jersey cattle found in "Jersey sires with their tested daugh- 

 ters," published by the American Jersey Cattle Club. In this 

 work seven day milk production is shown to be a logarithmic 

 curve and not a straight line. 



It remains to be shown that this same type of curve des- 

 cribes a true random sample of the Jersey breed for a longer 

 milk period. Data of this sort are important for several reasons 

 chief among which, both practically and theoretically, is the 

 necessity of having suitable correction factors for age to allow 

 comparison of milk records at different periods in the lives of 

 different cows. Toward the solution of this problem the fol- 

 lowing facts are necessary; what correlation exists between age 

 and milk production; is this correlation sufficient so that it must 

 be taken into account in considering records of different cows 

 at different ages; what is the equation of the regression line 

 between these two variables. Table 3 furnishes the data neces- 

 sary for this study. 



The correlation and its accompanying constants for these 

 two variables are shown in Table 4. 



TABLE 4. 



Constants Measuring the Association between Amount of Eight 

 Months Milk Produced and Age at Test of Jersey Cows. 



r 



V 



r] — r 



77 s — r 2 



0.2596±.0151 



0.4283±.0132 



0.1689±.0201 



0.1161+.0108 



This table makes clear several facts concerning the influence 

 of age on milk production. The correlation of -fo.2596zb.oi51 

 shows that age at test and milk production are significantly 

 correlated variates. Taken in conjunction with the correlation 

 ratio it shows clearly that age of the cow at commencement of 

 test must be considered in comparing the records of different 

 cows if the conclusion from the comparison is to be valid. The 

 value of the correlation ratio -l-0.4283ziz.0132 is considerably 



