Ecological Correlations and the Behavior of Individuals*

Abstract

AN INDIVIDUAL CORRELATION is a correlation in which the statistical object or thing described is indivisible. The correlation between color and illiteracy for persons in the United States, shown later in Table I, is an individual correlation, because the kind of thing described is an indivisible unit, a person. In an individual correlation the variables are descriptive properties of individuals, such as height, income, eye color, or race, and not descriptive statistical constants such as rates or means. In an ecological correlation the statistical object is a group of persons. The correlation between the percentage of the population which is Negro and the percentage of the population which is illiterate for the 48 states, shown later as Figure 2, is an ecological correlation. The thing described is the population of a state, and not a single individual. The variables are percentages, descriptive properties of groups, and not descriptive properties of individuals. Ecological correlations are used in an impressive number of quantitative sociological studies, some of which by now have attained the status of classics: Cowles’ “Statistical Study of Climate in Relation to Pulmonary Tuberculosis”;1 Gosnell's “Analysis of the 1932 Presidential Vote in Chicago,”2 Factorial and Correlational Analysis of the 1934 Vote in Chicago,”3 and the more elaborate factor analysis in Machine Politics;4 Ogburn's “How women vote,”5 “Measurement of the Factors in the Presidential Election of 1928,”6…