Power and Sample Size in Multilevel Linear Models

Sample size determination in multilevel designs requires attention to the fact that statistical power depends on the total sample sizes for each level. It is usually desirable to have as many units as possible at the top level of the multilevel hierarchy. Some formulas are given to obtain insight in the design aspects that are most influential for standard errors and power. Keywords: power; statistical tests; design; multilevel analysis; sample size; multisite trial; cluster randomization