The indicators most commonly used to measure SES come from Duncan's (1961) Socioeconomic Index (SEI), a composite of occupational prestige, income, and education. Duncan used data from North and Hart's study of 1949 occupational prestige and census data to conduct the first correlational study of the statistical relationship between education, income, and occupation. Duncan focused on white males with at least a high school education and income of $3,500 dollars or more in 1949, and found correlations among income, public-ranking of occupational prestige, and educational level of around 0.75. The study did not report whether the index included a sample of ethnic minorities.