Some diagnosticians may hesitate to use the Binet scale because of the criticisms it has received. Yerkes and Bridges state: “Indeed, we feel bound to say that the Binet scale has proved worse than useless in a very large number of cases” (_226_, p. 94). So far as this objection arises from the attempt to use the descriptions of the borderline of feeble-mindedness published with Binet scales, it will meet with a wide response. The difficulty is hardly less, as I shall show, with other scales. The definition of the borderline is certainly the vital point with any objective method for aiding diagnosis. Only by improving methods for determining the borderline can this weakness be attacked. The central contribution of this paper is directed, therefore, to this problem of the interpretation of the borderline, so that objective scales may be made more reliable for purposes of diagnosis. In Part Two I have added an intensive discussion of the measurement of development and a comparison of the different objective methods for describing the borderline. This may well be omitted by those who are not interested in the technical aspects of these questions. To those who care only for accounts of individual lives, let me say that I am contributing nothing herein to that important field which has been covered in authoritative form by Dr. Healy (_27_) and by Dr. Goddard (_112_). They will find instead, I hope, the fascination of figures, a picture book in which probability curves take the place of photographs and biographies, in which general tendencies are evaluated and attention is focussed upon the problem of properly diagnosing deficiency and upon plans for the care of the feeble-minded, whether they be potential or actual delinquents. ----- Footnote 1: Numbers in parenthesis indicate the references in the bibliography at the close of the book. Footnote 2: Those concerned with other features of the Binet scale will find an admirable bibliography by Samuel C. Kohs, Journal of Educational Psychology, April, May and June, 1914, and September, October, November, and December, 1917. Other references are contained in the Bibliography by L. W. Crafts (_9_). PART ONE PRACTICAL CONSIDERATIONS CHAPTER II. THE FUNCTIONS OF A SCALE IN DIAGNOSIS A. THE MEANING OF INTELLECTUAL DEFICIENCY.