Sample size calculator (Deff approach)

This spreadsheet implements the Design Effect (Deff) approach for sample size planning in random-intercept linear mixed models with repeated measures or clustered data. It allows users to translate independence-based sample size requirements into multilevel designs by specifying whether the primary effect of interest is defined at Level-1 (within-unit) or Level-2 (between-unit), and by exploring the impact of the intraclass correlation coefficient and number of observations per unit. The tool is intended to accompany the fact sheet “Sample size for LMMs using Design Effect” and to support transparent calculation, sensitivity analysis, and practical study planning.