s Medicine_ 48.1 49.0
Word Building 20.5 20.1
Logical Memory, _Costly Temper_ 64.0 69.6

[Illustration: FIGURE XIX--FREQUENCY SURFACES--COMPARING FOURTH GRADE
WITH HIGH SCHOOL

The numbers along the base represent mental age; those at the left, the
number of pupils of the respective ages.]

It may be thought that a child's success in school branches is a
sufficient measure of his ability and that no special mental
measurements are needed. This is a mistake. Many factors contribute to
success in school work. Ability is only one of these factors, and should
be specially and independently determined by suitable tests. Children
may fail in school branches because of being poorly started or started
at the wrong time, because of poor teaching, sickness, moving from one
school to another, etc. On the other hand, children of poor ability may
succeed at school because of much help at home. Therefore special mental
tests will help in determining to what extent original mental ability
is a factor in the success or failure of the different pupils.

As far as possible, the children of the same grade should have about the
same ability; but such is seldom the case. In a recent psychological
study of a school system, the author found wide differences in ability
in the same grade. The distribution of abilities found in the fourth
grade and in the high school are shown in Figure XIX. It will be seen
that in the fourth grade pupils are found with ability equal to that of
some in the high school. Of course to some extent such a condition is
unavoidable, for a pupil must establish certain habits and acquire
certain knowledge before passing from one grade to another. However,
much of the wide variation in ability now found in the same grade of a
school could be avoided if the teacher had accurate knowledge of the
pupils' abilities. When a teacher learns that a child who is doing
poorly in school really has ability, she is often able to get from that
pupil the work of which he is capable. It has been demonstrated by
experience that accurate measures of children's abilities are a great
help in gradation and classification.

A knowledge of mental differences is also an aid in the actual teaching
of the children. The instance mentioned at the close of the last
paragraph is an example. A knowledge of the differences among the mental
functions of the same pupil is especially helpful.