248 ROMMEL AND PHILLIPS— INHERITANCE IN 



[October 19, 



rowed was recorded. The date of farrowing of the dam was 

 also recorded so that a card similar to the one illustrated above 

 for the '' American Poland China Record " for 1902 would 

 now read : 



The cards were then arranged according to the age of the mother 

 at the time the dam named on the card was farrowed. This division 

 is necessarily rough, but the cards were arranged according to the 

 year of the birth of the dam, 1901, 1902, etc. Naturally the sows 

 placed in any one year are not all exactly of the same age, but this 

 rough segregation is enough to eliminate the factor of age as far 

 as is practical and necessary in this work. 



The method for calculating the correlation between mother and 

 daughter is given by Davenport (1904), ''Statistical Methods," 

 pp. 44-47, using the modification adopted by Yule (1897), which is 

 expressed in the formula 



Coefficient of correlation 



{r) == -- — -—^M -T-TT 

 n (7 a 



in which .r' and x" are the deviations from an assumed integral 

 mean of the subject and relative classes respectively, / is the fre- 

 quency of the several combinations, n, total number of individuals, 

 ?^/ and z\'\ the differences between assumed and real means of the 

 subject and relative classes, and a' and a", the standard deviations 

 of the subject and relative classes respectively. The standard devia- 

 tion is obtained from the formula 



•=>! 



'5(^V) 



n 



and is also equal to V^'o — v^. For a further discussion of these 



