POSTNATAL GROWTH IN THE ALBINO EAT 7 



where d represents the deviation from the mean, and n the num- 

 ber of variates. The standard deviation is therefore a concrete 

 number which serves to measure the dispersion from the mean. 

 If the variates tend to diverge greatly from the mean, the stand- 

 ard deviation will of course be large; while if they are concen- 

 trated closely around the mean, the standard deviation will be 

 small. It is further clear that the standard deviation is not 

 dependent upon the number of cases, but merely upon the 

 manner of their distribution. 



Since the standard deviation is a concrete number, however, 

 it will in general vary with the absolute size of the variate. 

 It is therefore often difficult to judge the relative variability of 

 different objects by comparing their standard deviations, and 

 where different units of measurement are used the comparison is 

 meaningless. On this account the coefficient of variation is pref- 

 erable for measuring the relative magnitude of variations, since 

 it is independent of the size or character of the unit of measure- 

 ment. 



It is of course obvious that from the statistical point of view 

 the number of observations is small and the results cannot be 

 considered final. Even although the probable errors are rela- 

 tively large, however, it will be found that certain conclusions of 

 importance may be drawn with a fair degree of certainty; and 

 others, more or less strongly indicated, may point the way to 

 further investigation with more adequate data. 



To economize space, the individual data are not included in 

 the tables, but only the averages and ranges are given. The 

 cards containing the original individual data are, however, 

 deposited in The Wistar Institute of Anatomy in Philadelphia 

 whence they may be obtained if desired by anyone interested. 



