The practical consideration of these borderlines in Part One and their location on the test scale emphasizes that a test diagnosis is only symptomatic, that the suggested borderlines on the Binet scale are determined from limited data which may not be verified in other communities, that the scale itself is imperfect, and that the results should be checked by other tests, especially by the school retardation, a new example of which is given for the Minneapolis delinquents. The plan of the percentage method of describing the borderlines readily allows for adjustment to more complete data or better developmental scales. The alternative to the use of a test record as symptomatic of deficiency is dependence upon the history of the case or physical signs, such as are found among Mongolians, cretins, epileptics, etc. These signs have been found among only about 13% of the deficient children (_141_). Expert opinion given on the history of the case is clearly less reliable than such opinion checked by even a crude objective test standard. In Part Two of this study the theoretical background for the percentage definition is compared with that of other quantitative definitions on the basis of the conceptions of mental measurement and mental development. 3. In attempting to suggest methods for diagnosis and control, which our summary of the scientific data makes necessary, we shall be led beyond the evidence presented in this study. To those to whom these suggestions may seem remoted from the foregoing pages, it may be said that they are the result not only of a review of the available research work, but also an outcome of several years observation of the practical handling of this problem both in this country and abroad. In that study I was led to visit several scores of institutions and schools for delinquent or deficient children in Austria, England, France, Germany, Italy and Switzerland. The methods suggested below for the case of the deficient delinquent are only modifications of what has been observed in actual operation. An adequate diagnosis of deficiency involves not only the accurate knowledge of the present mental condition of the individual, but an understanding of the causes of that condition. This requires a complete family and social history of the individual and a knowledge of the medically removable handicaps. It would seem, therefore, that such a diagnosis may be best made by a commission which shall include a physician as well as a psychologist, or else by an _expert in mental development_ who is provided with adequate facilities and assistance for discovering other handicaps than innate incapacity. For the group of uncertain and conative cases a final diagnosis should, if possible, be made only after prolonged observation in a temporary home school.