called the statistical
method; it is used in many practical matters, such as life insurance,
but its application to the facts of life, mind, variation, evolution,
etc., is only begun. Its neglect in psychology is one of the crying
defects of much recent work. Its use in complicated problems involves
a mathematical training which people generally do not possess; and its
misuse through lack of exactness of observation or ignorance of the
requirements is worse than its neglect.

Another result came out in connection with these experiments on
memory, which, apart from its practical interest, may serve to show an
additional resource of experimental psychology. In making up the
results of a series of experiments it is very important to observe the
way in which the different cases differ from one another. Some cases
may be so nearly alike that the most extreme of them are not far from
the average of them all; as we find, for example, if we measure a
thousand No. 10 shot. But now suppose we mix in with the No. 10 some
No. 6 and some No. 14, and then take the average size; we may now get
just the same average, and we can tell that this pile is different
from the other only by observing the individual measurements of the
single shot and setting down the relative frequency of each particular
size. Or, again, we may get a different average size in one of two
ways: either by taking another lot of uniform No. 14 shot, let us say,
or by mixing with the No. 10 a few very large bullets. Which is
actually the case would be shown only by the examination of the
individual cases. This is usually done by comparing each case with the
average of the whole lot, and taking the average of the differences
thus secured--a quantity called the "mean variation."

In the case of the experiments with the squares, the errors in the
judgments of the students were found to lie always in one direction.
The answers all tended to show that they took, for the one originally
shown, a square which was really too large. Casting about for the
reason of this, it was considered necessary to explain it by the
supposition that the square remembered had in the interval become
enlarged in memory. The image was larger when called up after ten or
twenty minutes than it was before. This might be due to a purely
mental process; or possibly to a sort of spreading-out of the brain
process in the visual centre, giving the result that whenever, by the
revival of the brain pro