Abstract
AbstractAn ecologic study focuses on the comparison of groups rather than individuals; thus, individual‐level data are missing on the joint distribution of variables within groups. The purpose of an ecologic analysis may be to make biologic inferences about effects on individual risks or to make ecologic inferences about effects on group rates. Ecologic study designs may be classified on two dimensions: (a) whether or not the primary exposure is measured; and (b) whether subjects are grouped by place, time, or place and time. Despite practical advantages of ecologic studies, there are several methodologic problems that severely limit causal inference, including ecologic and cross‐level bias, problems of confounder control, within‐group misclassification, lack of adequate data, temporal ambiguity, collinearity, and migration across groups. Even when the study objective is to estimate ecologic effects, individual‐level data are generally needed to make valid causal inferences, preferably using multilevel analysis.
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