Ecological regression

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Ecological regression is a statistical method that uses group data, not individual data, to estimate how different groups vote. For example, researchers can look at counties and record the county’s Democratic vote share (D) and the share of Catholics (C). They fit a simple linear model D = a + bC. If the result is D = 0.22 + 0.45C, the model estimates that all-Catholic counties (C = 1) would vote about 67% Democratic, while counties with no Catholics (C = 0) would vote about 22% Democratic.

The method helps infer how groups within a population may vote based on aggregate data, such as Catholics vs. non-Catholics or blacks vs. whites. It has been used in cases under the Voting Rights Act to study voting patterns. Like any model, it relies on assumptions and its conclusions should be interpreted with care.