Figure 1 shows the distribution of mental ages for 62 adults, including the 30 business men and the 32 high-school pupils who were over 16 years of age. It will be noted that the middle section of the graph represents the "mental ages" falling between 15 and 17. This is the range which we have designated as the "average adult" level. Those above 17 are called "superior adults," those between 13 and 15, "inferior adults." Subjects much over 15 years of age who test in the neighborhood of 12 years may ordinarily be considered border-line cases. [Illustration: FIG. 1. DISTRIBUTION OF MENTAL AGES OF 62 NORMAL ADULTS] The following method was employed for determining the validity of a test. The children of each age level were divided into three groups according to intelligence quotient, those testing below 90, those between 90 and 109, and those with an intelligence quotient of 110 or above. The percentages of passes on each individual test at or near that age level were then ascertained separately for these three groups. If a test fails to show a decidedly higher proportion of passes in the superior I Q group than in the inferior I Q group, it cannot be regarded as a satisfactory test of intelligence. On the other hand, a test which satisfies this criterion must be accepted as valid or the entire scale must be rejected. Henceforth it stands or falls with the scale as a whole. When tried out by this method, some of the tests which have been most criticized showed a high degree of reliability; certain others which have been considered excellent proved to be so little correlated with intelligence that they had to be discarded. After making a few necessary eliminations, 90 tests remained, or 36 more than the number included in the Binet 1911 scale. There are 6 at each age level from 3 to 10, 8 at 12, 6 at 14, 6 at "average adult," 6 at "superior adult," and 16 alternative tests. The alternative tests, which are distributed among the different groups, are intended to be used only as substitutes when one or more of the regular tests have been rendered, by coaching or otherwise, undesirable.[17] [17] See p. 137 _ff._ for explanations regarding the calculation of mental age and the use of alternative tests. Of the 36 new tests, 27 were added and standardized in the various Stanford investigations. Two tests were borrowed from the Healy-Fernald series, one from Kuhlmann, one was adapted from Bonser, and the remaining five were amplifications or adaptations of some of the earlier Binet tests. Following is a complete list of the tests of the Stanford revision. Those designated _al._ are alternative tests. The guide for giving and scoring the tests is presented at length in Part II of this volume. _The Stanford revision and extension_