14 BULLETIN 1121, U. S. DEPARTMENT OF AGRICULTURE. 
of mating years (Table 14) and a is calculated from the above formu- 
lz, using the means in Table 14. 
The formula for the probable error of a percentage (p) is 0.6745 
oe P heey we In applying this to percentage born alive, the question 
arises as to whether n should be the number of individuals (in litters 
of 1 to 4) or the number of litters. In the former case -/n is about 
50 per cent larger than in the latter. A compromise is in line with 
the system of indices which has been used. Accordingly, n has been 
taken as the number of individuals born in litters of 1 to 4, but the 
resulting estimate of the probable error has been increased by 25 per 
cent, i. e., PE=1.25 x 0.6745 = GD 
The percentage raised of the young born alive has been dealt with 
similarly, n being the number born alive in litters of 1 to 4. In the 
case of the percentage raised of all young, n is again the total number 
born in such litters. 
Similar considerations apply in the case of the weights. The 
formula PE=1.25 x 0.6745 te has been used in each case, n being 
the total number born in litters of 1 to 4 in the case of the birth weight 
of all young, and merely the number raised in such litters in the other 
cases. The standard deviations for a given size of litter have been 
taken as 14.76 grams for birth weight of all young, 13.62 grams for 
birth weight of young raised, 1.285 grams per day for gain, and 49.53 
grams for 33 days’ weight. These figures apply to litter means and 
follow from the following determinations based on an analysis of 
3,009 litters of inbreds and controls: 
Birth weight (total)___.__-.- oy’ = 19.49 grams. 
Tsp= — 0.6580 +0.066. 
17p/ = 14.76 grams. 
Birth weight (raised) - - _- - - - o3= 18.09 grams. 
fg,= — 0.6580 + 0.066. 
107p = 13.62 grams. 
Gain vied _ soiiderunsAtsoloes 408 Gorams(33 aye): 
To. = — 0.38807 +0.0105. 
10¢ = 42.42 grams = 1.285 grams perday. 
Weight at 33 days.____.__- o,,= 57.08 grams. 
fpg= +0.5326. 
9q= 49.53 grams. 
The formula for standard deviation for a given size of litter (L) 
is of the type 
we = op VI—? sy. 
The formula for ,c3, is derived from the equation 
L733. aa LOB" ate L0"G te 21.08 °10G'1LT'BG- 
