A. COMPARISON OF UNITS AND SCALES FOR MEASURING INDIVIDUAL 254 DIFFERENCES (a) Equivalent Units of Ability When the Distributions 254 are Normal (b) The Year Unit of the Binet Scale 260 (c) Is Tested Capacity Distributed Normally? 267 (d) Equivalent Units of Development When the Form of 275 Distribution is Uncertain B. THE CURVES OF MENTAL DEVELOPMENT 279 (a) The Significance of Average Curves of Development 280 (b) Changes in the Rate of Development 290 (c) The Question of Earlier Arrest of Deficient Children 294 Chapter XIV. QUANTITATIVE DEFINITIONS OF THE BORDERLINE 304 A. DIFFERENT FORMS OF QUANTITATIVE DEFINITIONS 304 B. COMMON CHARACTERISTICS OF QUANTITATIVE DEFINITIONS 308 C. PRACTICAL ADVANTAGES OF THE PERCENTAGE METHOD 311 D. THEORETICAL ADVANTAGE OF THE PERCENTAGE METHOD WITH CHANGES 317 IN THE FORM OF THE DISTRIBUTIONS BIBLIOGRAPHY ON TESTED DELINQUENTS 324 Other References Cited 329 APPENDICES 344 INDEX 353 LIST OF TABLES AND FIGURES TABLES I. Age distribution of deaths in the general population and of 30 feeble-minded in institutions II. Mortality of institutional deficients in the United States 31 compared with the general population III. Test borderlines with randomly selected Minneapolis 89 15-year-olds IV. Results with the Binet tests for mental ages XI and XII 98 (1908 series) V. Percentages of mentally retarded children as tested with the 106 Binet 1908 Scale VI. Mental retardation of children as tested with the Binet 1911 111 Scale VII. Borderlines with the Point Scale 115 VIII. Test ages of the Glen Lake group of delinquent boys 124 IX. Intellectual development relative to life-ages and school 125 positions among the delinquent boys of Glen Lake X. Binet 1911 tests of boys consecutively admitted to the 151 Detention Home at Thorn Hill, Allegheny County XI. Frequency of tested deficiency among over 9000 delinquents 159 XII. Age and grade distribution of elementary school pupils in 178 Minneapolis XIII. School retardation of Minneapolis delinquents and elementary 179 school pupils XIV. Indices of frequency and amount of school retardation for 183 Minneapolis delinquents and elementary school pupils