The author evaluates mental testing methods and proposes a quantitative definition of intellectual deficiency, arguing that deficiency is a degree rather than a different kind. The work explains adapting percentage cutoffs to the Binet and other developmental scales, identifies borderline regions for immature and mature examinees, and compares test results with school retardation and institutional data. Drawing on delinquent samples and school records, it analyzes correlations between tested deficiency and delinquency, discusses alternative diagnostic checks, and considers implications for social care and testing practice.