ency surface, which is constructed as
follows: Draw a base line A B, and on this line mark off equal distances
to represent the various weights. At the left end put the number
representing the lightest men and at the right the number representing
the heaviest men; the other weights come in between in order. Then
select a scale; we will say a millimeter in height above the base line
represents one person of the weight represented on the base, and in
drawing the upper part of the figure, A C B, we have but to measure up
one millimeter for each person weighed, of the weight indicated below on
the base.

[Illustration: FIGURE V--FREQUENCY SURFACE--WEIGHT
The solid line represents men, the broken line, women.]

A study of this frequency surface shows a tendency for people to be
grouped about the central tendency or average. There are many people of
average weight or nearly so, but few people who deviate widely from the
average weight. If we measure people with reference to any other
physical characteristic, or any mental characteristic, we get a similar
result, we find them grouped about an average or central tendency.

=Mental Differences.= Just as we differ physically, so also we differ
mentally, and in the various aspects of our behavior. The accompanying
diagram (Free Association) shows the distribution of a large number of
men and women with respect to the speed of their flow of ideas. When men
and women are measured with respect to any mental function, a similar
distribution is found.

[Illustration: FIGURE VI--FREQUENCY SURFACE--FREE ASSOCIATION
Solid line, men; broken line, women. The numbers below the base
represent the number of words written in the Free Association test, and
the numbers at the left represent the number of people making the
respective scores.]

An interesting question is whether our mental differences have any
relation or connection with one another. If one mental characteristic is
of high order, are all the others of high order also? Does a good memory
indicate a high order of attention, of association, of imagination, of
learning capacity? Experiments show that mental characteristics have at
least some degree of independence. But the rule is that they generally
go together, a high order of ability in one mental function indicating a
high order of ability in at least some others, and a low order of
ability in one function indicating a low order in other functions.

However, it seems that ab