Ecological Studies

Ecological studies are most often based on existing data and rely on units of measurement that are groups of subjects rather than individuals. An example would be a study of the incidence rates of a disease where nutritional deficiencies are thought to be causally important in countries that vary in exposure to that nutritional deficiency. Grouping can be on the basis of any characteristic (e.g., geography, year of birth, etc.) but is usually socio-demographic. Migrant studies fall into this category (Marmot, 1989) but are more informative when conducted as case-control studies. Ecological studies are a useful starting point in preliminary analyses of research questions, often generating many testable hypotheses (Armstrong and Doll, 1975). Power calculations for ecological studies determine the number of groups required in each arm of the comparison, not the number of subjects.

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