pect those to do whose mental development is accelerated.
Correspondingly, those who tested _minus_ at the first test advanced
only about three fourths of a year in mental age during the
interval.[39]

[39] Otto Bobertag: "Ueber Intelligenz Pruefungen," in _Zeitsch. f.
Angew. Psychol._ (1912), p. 521 _ff._

Our own results with a mixed group of normal, superior, dull and
feeble-minded children agree fully with the above findings. In this case
the two tests were separated by an interval of two to four years, and
the correlation between their results was practically perfect. The
average difference between the I Q obtained in the second test and that
obtained in the first was only 4 per cent, and the greatest difference
found was only 8 per cent.[40]

[40] See _The Stanford Revision and Extension of the Binet-Simon Scale
for Measuring Intelligence_. (Warwick and York, 1916.)

The repetition of the test at shorter intervals will perhaps affect the
result somewhat more, but the influence is much less than one might
expect. The writer has tested, at intervals of only a few days to a few
weeks, 14 backward children of 12 to 18 years, and 8 normal children of
5 to 13 years. The backward children showed an average improvement in
the second test of about two months in mental age, the normal children
an average improvement of little more than three months. No child varied
in the second test more than half a year from the mental age first
secured. On the whole, normal children profit more from the experience
of a previous test than do the backward and feeble-minded.

Berry tested 45 normal children and 50 defectives with the Binet 1908
and 1911 scales at brief intervals. The author does not state which
scale was applied first, but the mental ages secured by the two scales
were practically the same when allowance was made for the slightly
greater difficulty of the 1911 series of tests.[41]

[41] Charles Scott Berry: "A Comparison of the Binet Tests of 1908 and
1911," in _Journal of Educational Psychology_ (1912), pp. 444-51.

We may conclude, therefore, that while it would probably be desirable
to have one or more additional scales for alternative use in testing the
same children at very brief intervals, the same scale may be used for
repeated tests at intervals of a year or more with little danger of
serious inaccuracy. Moreover, results like those set forth above are
important evidence as to the validity of the test method.