mental age, but the failures
below and the successes above balance each other in the average child,
so that he comes out with a mental age equal to his chronological age.

[Footnote: The Binet scale, it must be understood, is an instrument
of precision, not to be handled except by one who has been
thoroughly trained in its use. It looks so simple that any student
is apt to say, "Why, I could give those tests!" The point is that he
couldn't--not until he knew the tests practically by heart, not till
he had standardized his manner of conducting them to agree perfectly
with the prescribed manner and till he knew how to score the varying
answers given by different children according to the scoring system
that goes with the tests, and not till, by experience in handling
children in the tests, he was able to secure the child's confidence
and get him to do his best, without, however, giving the child any
assistance beyond what is prescribed. Many superior persons have
looked down on the psychological examiner with his (or her)
assortment of little tests, and have said, "Certainly no special
training is necessary to give these tests. You simply want to find
out whether the child can do these stunts. I can find out as well as
you." They miss the point altogether. The question is not whether
the child can do these stunts (with an undefined amount of
assistance), but whether he _does_ them under carefully prescribed
conditions. The child is given two, three or four dozen chances to
see how many of them he will accept; and the whole scale has been
standardized by try-out on many children of each age, and so adapted
that when given according to instructions, it will give a correct
measure of the child's mental age. But when given by superior
persons in ignorance of its true character, it gives results very
wide of the mark. So much by way of caution.]

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If a child's mental age is the same as his chronological age, he is
just average, neither bright nor dull. If his mental age is much above
his chronological, he is bright; if much below, dull. His degree of
brightness or dullness can be measured by the number of years his
mental age is above or below his chronological age. He is, mentally,
so many years advanced or retarded.

Brightness or dullness can also be measured by the _intelligence
quotient_, which is employed so frequently that it is customarily
abbreviated to "