Mutually exclusive subgroups of the target population based on important characteristics or variables.
Kleinbaum, D., L. Kupper, and H. Morgenstern. 1982. Epidemiologic Research: Principles and Quantitative Methods, Belmont, CA : Lifetime Learning Publications.
Gliner, J.A. and G.A. Morgan. 2000. Research Methods in Applied Settings, Mahwah, NJ: Lawrence Erlbaum Associates.
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