We have thus been able to restate the problem of the deficient delinquent more conservatively and to modify some of the current conceptions. This enables us to direct our efforts more intelligently, with greater foresight, and more hope of success. 2. A still broader outcome of this interpretative study is to increase the precision of the test scales for use in the diagnosis of social deficiency. This has been accomplished by an extended reconsideration of the borderlines of deficiency on test scales, particularly the Binet scale. A percentage definition of tested deficiency is suggested for determining the borderline below which an individual may be presumed to be so deficient as to justify isolation, and for setting off a distance above this on the scale for which the test diagnosis of social deficiency should be regarded as uncertain. By this means it is hoped that the developmental scale may be made safer and more useful as an instrument for diagnosing feeble-mindedness. A quantitative definition for tested deficiency has its main justification in its success in discovering social deficients and in predicting social failure. With this in mind the percentages suggested as representing the social deficients or uncertain cases in the community were chosen after a careful search through the evidence as to the success of children who had been in special classes or institutions and an extensive résumé and analysis of the best expert estimates of the frequency of social deficiency. The conclusion was that these percentages may tentatively be placed so that those who would at 15 years of age be in the lowest 0.5% in tested ability among a randomly selected group, may be presumed to be so deficient as to justify isolation. Above these the next 1.0% may be regarded as uncertain, since the bulk of them would require some supervision or guardianship during life. These two borderlines have then been located on the Binet scale for both the immature and the mature so far as possible from the available data. In particular these borderlines for the mature have been found for the first time on the basis of a randomly selected group. Besides the records of Minneapolis delinquents these Binet borderlines for a typical random population of 643 15-year-olds is the main contribution of new data in the study.