This monograph presents an exhaustive study based on the results of Stanford-Binet examinations given to 827 patients at the Worcester State Hospital during the years 1929 to 1933. The study includes an attempt to determine (a) the general test level of various diagnostic groups at the hospital and (b) the relative performance of the different groups in specific items in the test. Extraneous factors which might modify the results, such as disparities in age, educational level or occupation, were excluded by selecting groups of individuals as closely alike as possible in these respects; this left the diagnostic factor the essential criterion.