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for in this instance we have considerable approximation to uniformity (if environment. For 
261 normal children examined by the Binet-Simon method by Dr Jaederholm, I tind the 
coefficient of variation in intelligence as measured in mental years to be 19-4:76. For 420 
children in two schools I find a coefficient of variation in general intelligence of 21-986, and for 
1725 children in eight schools I find a coefficient of variation in terms' marks of 2;V133 
These are somewhat greater than the variability obtained for the orphanage children, ljut do 
not show the great increase some might anticipate from variety in home and school training, 
and the increase of the last two results may Ije .■^olely due to the different standards imposed by 
the judgments of a variety of teachers instead of, as in Jaederholm's and <:>ur present cases, 
an identical series of tests made by a single psychologist. The noteworthy v;du?, however, lies 
in the correlation of -508. The values obtained for 12 cases of physical characters in siblings 
{Biometrika, Vol. ii. p. 387) have exactly this value for their mean. No stress can, of course, be 
laid on the absolute identity considering the smalhiess of the present series, but much stress 
may be laid on the api)roximati(3n of the two results. 
But the present data are of further interest— although they are so slender —when we compare 
the results to be obtained from them with those for a far longer series of pairs of siblings 
obtained by the method of "broad-categories.'' This series is also formed from pairs of siblings 
who are children. They belonged to a great variety of schools taken throughout Great Britain. 
Every variety of environment, every variety of educational and home training is therefore 
included. Accordingly if the intellectual resemblance of siblings were the result or largely 
the result of difterential treatment, we ought to anticipate a great increase of ccirrelation in this 
material over that of the material drawn from the Californian orphanages. We have also the 
possibility of obtaining light on two further pi'oblems : 
(i) Whether the method of "broad-categories" really does give results markedly inferioi- 
to the Binet-Simon method of direct quantitative measurement. 
(ii) What is the approximate value of the "mentace" or unit of intelligence in terms of a 
unit obtained from a Binet-Simon test. 
The definitions of the "broad-categories" used by the Galton Laboratory in its intelligence 
investigations have already been published in this journiil*, and a "mentace" has been defined 
as the of the range which limits the category " Intelligentt." Now if we compare the 
two series, the one determined by "broad-categories " and the other by the Binet-Simon test for 
the total frequencies uji to the beginning and up to the end of the range " Intelligent," we shall 
have a first approximation — on the assumption that l;)oth sei-ies are measuring the same general 
intelligence character and both approximate to normal distributions — to the absolute value of a 
"mentace." I find that my mentace is equal to -1604 of Dr Gordon's intelligence quotient 
units, or with the average age of 10--2 (which appears to have been that of her children) it equals 
six days about of mental growth of children at this age. Roughly we might say tliat a mentace 
is equal to about a week's mental growth at the age of ten years. In estimating the meaning of 
this statement we must remember that mental growth is very rapid at this age J. 
As the American data pool children of both sexes I have for purposes of comparison done tlie 
same. The following table represents my material for 5602 children in 2801 pairs, each pair 
being entered either way so as to produce a symmetrical table. 
* Biometrlka, Vol. viii. p. 93. 
t Biomctrika,\o\. v. p. 109. 
+ The reader will of course avoid the conclusion that the mentace is an intelligence unit varying 
with age. It is the time rate of growth of intelligence which varies with age, and we must state 
a particular age in evaluating the mentace in terms of growth of intelligence. 
