The Variation of Milk Secretion with Age. 



53 



production with age has already been shown, by the work of 

 this laboratory, to conform to that group of curves described by 

 a logarithmic function. 



The standard deviation varies between 818.9 pounds at the 

 age of 2 to 3 years to 1325.3 pounds at the age of 6 to 7 years. 

 The rise of the standard deviation is more direct than is the 

 rise of the mean curve. From the. maximum the curve remains 

 parallel with the base line for some years when it again drops 

 to a lower deviation at the highest age at test. This rise and fall 

 is significant as judged by the probable errors of the difference. 

 The difference of the maximum standard deviation at 6 to 7 

 years from that at 2 to 3 years is 506.4 pounds of milk where 

 the probable error is 50.8 pounds or the difference is practically 

 10 times the probable error and the probability that this differ- 

 ence did not come from random sampling is considerably greater 

 than 1,000,000,000 to 1. The difference between the maximum 

 standard deviation and that at 10 years and above is 248.0 

 pounds and the probable error is 67.5 pounds or 3.68 times the 

 probable error and the probability that this difference did not 

 come from random sampling is 75.7 to 1. From these above 

 facts the conclusion seems justified that milk production varies 

 with age at test in such a way that the absolute amount of the 

 variation in production from one cow to another is least at the 

 early age, increases rapidly to a maximum at about 5 years and 

 6 months, remains at this maximum for about four years then 

 falls again toward the later ages. 



The coefficient of variation for the milk production of the 

 successive ages are all high as compared with those already 



'TABLE 2. 



Comparison Data for Coefficients of Variation of Amount of 



Secretion. 



Organism 



Secretion 



Domestic Fowl 



(Barred Plymouth Rock) 

 Cattle British Holstein 

 Cattle Ayrshire 

 Cattle Jerseys 



Annual Egg Production 



Gallons of milk 

 Gallons of milk 

 Pounds of milk 



Coefficient 

 of Variation 



34.21±.37# 



25.72±.37 

 24.1S+.50* 

 25.56-K31 



Source of 

 Data 



Pearl 



Gavin 

 Vigor 

 This paper 



# Probable error calculated by author from total given in paper. 

 *Calculated by the author from the means and standard deviation as given by 

 Mr. Vigor. , • ■ 



