for this is that in a complex system, such as dealt 
with here, it is rarely possible to conclusively 
isolate cause and effect. 
a. Average growth curves. For rats between 10 
and 460 days of age there were 462 measurements 
of weight in grams and body length in millimeters 
for females, and 680 for males. The age range 
was divided into 28 class intervals so that there 
was a minimum of 10 records for a class interval. 
Three figures for each interval were derived; (1) a 
mean; (2) a “maximum” — this was the mean of 
the largest one-fifth of the measurements; (3) a 
“minimum” — this was the mean of the smallest 
one-fifth of the measurements. Average growth 
curves (fig. 130 to 133) were developed for these 
by the following procedure recommended by 
Professor W. G. Cochran of Johns Flopkins 
University. 
K—y = 10 [a—b(x— 40)] 
where : 
K= adult weight (This I assigned as the mean 
weight between 340 and 460 days of age.) 
y= weight at a given age 
*=age in days 
o — log (K—yw) 
b=a - log (K—y m ) 
Therefore y — 10 (a — b(x — 40)] 
— K — Antilog [a — b(x — 40)] 
Since a new pattern of growth usually ensued 
shortly after weaning (which for the present 
purpose was taken as 40 days) the ordinate intercept 
occurs at 40 days of age. Therefore, in calculating 
y, x — 40 must be utilized instead of x. The 
observed data was plotted on semilog paper. The 
log ordinate was K — y and the abscissa was age in 
days. For these mean data, so plotted, a straight 
600 
500 
g 400 
< 
cr 
e> 
300 
x 
o 
Ll) 
£ 
200 
100 
0 100 200 300 400 500^ 
AGE IN DAYS 
Figure 130. — Growth in weight of males. The maximum curve is that for the upper 20 percent of the records in each age 
class interval. The minimum curve is that for the lower 20 percent of the records in each age class interval. K's represent 
the mean adult weight at 391 days (i.e. all records between 340 and 450 days of age). Note that the rats with the 
lowest mean adult weight also grew slower as indicated by the 62-day differential between them and the heaviest rats 
in the age of attainment of the mean adult weight. 
217 
676-768 0—63 15 
