This hypothesis can be tested by
a variety of mental measurements. S. A. Courtis' investigation of the
arithmetical abilities of the children in the schools of New York City
will be a good beginning. He measured the achievements of pupils in
responding to eight tests, which were believed to give a fair idea of
the pupil's capacity for solving simple arithmetical problems. The
results were, on the average, similar to the result he got in a certain
eighth-grade class, whose record is shown in Fig. 9. It is evident that
some of the children were good in arithmetic, some were poor in it; the
bulk of them were neither good nor bad but half way between, or, in
statistical language, mediocre.
[Illustration: VARIATION IN ABILITY
FIG. 9.--Diagram to show the standing of children in a single
class in a New York City school, in respect to their ability in
arithmetic. There are wide divergences in the scores they made.]
The literature of experimental psychology and anthropology is crammed
with such examples as the above. No matter what trait of the individual
be chosen, results are analogous. If one takes the simplest traits, to
eliminate the most chances for confusion, one finds the same conditions
every time.
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