[39] Otto Bobertag: "Ueber Intelligenz Prüfungen," 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. INFLUENCE OF SOCIAL AND EDUCATIONAL ADVANTAGES. The criticism has often been made that the responses to many of the tests are so much subject to the influence of school and home environment as seriously to invalidate the scale as a whole. Some of the tests most often named in this connection are the following: Giving age and sex; naming common objects, colors, and coins; giving the value of stamps; giving date; naming the months of the year and the days of the week; distinguishing forenoon and afternoon; counting; making change; reading for memories; naming sixty words; giving definitions; finding rhymes; and constructing a sentence containing three given words.