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Intraclass Correlation Values for Planning Group-Randomized Trials in Education

Northwestern University

E. C. Hedberg

University of Chicago

Experiments that assign intact groups to treatment conditions are increasingly common in social research. In educational research, the groups assigned are often schools. The design of group-randomized experiments requires knowledge of the intraclass correlation structure to compute statistical power and sample sizes required to achieve adequate power. This article provides a compilation of intraclass correlation values of academic achievement and related covariate effects that could be used for planning group-randomized experiments in education. It also provides variance component information that is useful in planning experiments involving covariates. The use of these values to compute the statistical power of group-randomized experiments is illustrated.

Key Words: intraclass correlation • cluster randomized trials • experiments • statistical power

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