8 MASS. EXPERIMENT STATION BULLETIN 



coefficient of variation was about 17 per cent. There was no difference in the 

 average time interval to standard egg weight in the 297 birds that completed 

 the year and the 86 birds that failed to complete the year. 



The coefficient of correlation is positive and of no significance, but the regres- 

 sion is non-linear. The correlation ratio of .3015, therefore, expresses the asso- 

 ciation between annual production and time to standard egg weight. This 

 constant shows that about 9 per cent of the variation in the time interval is due to 

 variation in annual egg production. In other words, heavy producers tend to 

 require more time to attain standard egg weight. 



Correlation Between Mean Annual Egg Weight and Time to Standard Egg Weight 



Mean annual egg weight was obtained for each bird by taking the average weight 

 of all eggs laid from October 1 to September 30 each year. The simple correlation 

 between annual egg weight and time to standard egg weight was calculated, with 

 the constants shown below: 



Number of birds 306 



Mean annual egg weight, grams 55.45 



Annual egg weight standard deviation ±2.59 



Mean time to standard egg weight, days 108.15 



Time to standard egg weight standard deviation +48.41 



Coefficient of correlation — .6532 + .0221 



The data show that the mean annual egg weight was 55.45 grams, or the 

 equivalent of 23.5 ounces to the dozen. Variability in egg weight was relatively 

 slight, showing a coefficient of variation of about 4.6 per cent. 



The coefficient of correlation between annual egg weight and time to standard 

 egg weight was negative and of rather high magnitude. On the basis of its value 

 squared, about 43 per cent of the variation in time required is due to variation in 

 annual egg weight. The regression in this case was strictly linear. The data 

 show that the time to standard egg weight very significantly affects the mean 

 egg weight for the year; and if the mean egg weight for the year is to be 56.7 

 grams or 24 ounces to the dozen, pullets must reach standard egg weight in about 

 85 days, as calculated by the regression coefficient. 



Multiple Correlation Between Time to Standard Egg Weight and Factors Affecting It 



This stud) - has demonstrated by means of the simple and the partial coefficients 

 of correlation that the following factors affect the time to standard egg weight: 

 hatching date, age at first egg, weight at first egg, winter clutch size, winter pause 

 duration, total days broody, and annual persistency. With the possible exception 

 of winter pause duration, all exert a significant effect on time to standard egg 

 weight. These constants are placed together in Table 1. 



Multiple correlation is resorted to in order to determine the combined effect of 

 these seven factors (Nos. 2, 3, 4, 5, 6, 7, 8 in Table 1). Calculations show R = .7641. 

 By squaring R, the value .5839 is obtained, which indicates that about 58 per 

 cent of the variation may be accounted for by variation in the seven factors 

 considered, leaving 42 per cent that appears to depend on undetermined hereditary 

 factors. In view of this fact it seems probable that time to standard egg weight 

 does depend upon hereditary factors and that poultry breeders can very likely 

 make progress in breeding for large egg size by selecting only female breeders that 

 attain standard egg weight in 75 days or less after the first pullet egg. 



