July 8, 1918 
Percentage of Fat in Cow's Milk 
69 
POSSIBLE SOURCES OF ERROR 
In some of the breeds, Jersey and Guernsey, a yearly test consists of 
any 365 consecutive days. This may cover parts of two lactation 
periods, which is not a serious objection, since the study is interested in the 
relation between the percentage of butter fat and yield of milk in groups 
of individuals and not in the individual herself. In other words, the 
question is to what extent, if any, do cows with large milk yield tend to 
show a low percentage of fat, or cows with a low milk yield to show a 
high percentage. The Ayrshire Association specifies both the amount of 
milk and butter fat necessary for entrance. In the other associations only 
the butter-fat yield is specified. Unless the requirements of the Ayrshire 
Association are in accordance with the natural relation of butter-fat and 
milk yield, one would expect to find abnormal results in such a selected 
group, which would not hold for Ayrshire cattle in general. This point 
will be treated more fully later in this paper. For the grade Jerseys, 
grade Holstein-Friesians, and cows unclassified as to breed, it should be 
pointed out that the populations are composed of a heterogeneous lot, 
and whatever results are found will apply only to such mixed popu- 
lations. 
ANALYSIS OF DATA 
JERSEY 
Tables I to IV show, in the form of correlation tables, the distri- 
bution of individuals with regard to the yield of milk and the percentage 
of fat. Table V contains all the tests of Jerseys regardless of age and 
was made by combining Tables I to IV. Text Table A summarizes the 
means, standard deviations, coefficients of variability of milk and fat, 
and the correlation between the percentage of fat and the yield for 
Jerseys of different ages, and for Jerseys, irrespective of age. 
TablS a. — Summary of results from a study of the correlation between the percentage of 
fat and yield of milk for ferseys 
[Fat in percentage; milk in pounds] 
Age. 
Num- 
ber of 
animals. 
Type of 
test. 
Mean. 
Standard 
deviation. 
Coefficient 
of varia- 
bility. 
Correlation. 
Years. 
s to 3 
3 to 4 
4tos 
5 and over 
All ages 
877 
411 
219 
634 
2, 141 
/Milk 
\Fat 
/Milk 
IFat 
/Milk 
\Fat 
/Milk 
\Fat 
/Milk 
\Fat 
6,475.0 ±28.9 
5. 425± 0.012 
7,325-0 ±45.8 
5. 401 ± 0. 019 
8. 043. 4 ± 60. 7 
5. 462± 0.024 
8.814.5 ±43-1 
5- 322 ± 0. 013 
7,491-4 ±25.0 
S.392± 0.008 
1,270- S ±20.5 
0. si7± 0. 008 
1.377.5 ±32.4 
0. 562 ± 0. 013 
1.332.5 ±42.9 
0. 533 ± 0. 017 
1,608.5 ±30.5 
0. 502 ± 0. 010 
1,718.0 ±17.7 
0. 525 ± 0. 00s 
19. 62±o. 33 
9- 52±o. IS 
18. 81 ±0. 46 
10. 4i±o. 25 
16. 57±o. 55 
9. 76±o. 32 
18. 2S±o. 35 
9. 44±o. 18 
22. 93iho. 25 
9. 74±o. 10 
0. 36o±o. 020 
0. 437±o. 027 
J~o.359± 0.040 
|— 0. 397±o. 023 
}— o-3S4±o. 013 
