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Ecological Regression Versus Homogeneous Units: A Specification Analysis

Writing more than twenty five years ago, W. S. Robinson assailed the assumption implicit in much empirical work that statistical measures computed for aggregate units—states, provinces, counties, cities, wards, school districts—could be used in place of corresponding measures for the individuals comprising these units. Robinson was not the first scholar to expose the pitfalls of naive inferences from group level data to individual level behavior. But he first brought this problem forcefully to the attention of practicing social scientists. Focusing on the Pearson product moment correlation coefficient (r), Robinson proved that an aggregate level coefficient need not be equal in value to the corresponding individual level coefficient. Blending empirical examples with mathematical demonstration, Robinson showed that when the individual level correlation between being a native American and being able to read was computed for state level percentages of natives and literates, its value changed from .118 to -.526. Thus an investigator relying on correlations computed for the American states would not even correctly assess the direction of the relationship between nativity and literacy. Coining new jargon, Robinson used the term “ecological fallacy” to describe a naive inference from the group to the individual level of analysis. The reverberations of Robinson’s work are still being felt by those interested in the past behavior of individuals. Historians without access to survey research and experimental techniques must rely on data that have already been collected. Because such data so often pertain to aggregate units, historians frequently must use cross-level inference to estimate the behavior of individuals. Having paid virtually no attention to the methodology of cross-level inference for almost twenty years after the publication of Robinson’s work, historians have suddenly discovered the “ecological fallacy.” Authors dread to see the term “ecological fallacy” scribbled in the margins of their work; to fall victim to this fallacy is to forfeit one’s claim to methodological legitimacy.