Abstract
When should researchers use linear mixed-effects models (LMMs) versus repeated-measures analysis of variance (RM-ANOVA) for analysing repeated-measures data? I first outline the conceptual and statistical differences between the two approaches: RM-ANOVA operates on aggregated difference scores, whereas LMMs model trial-level data using fixed and random effects to account for dependencies among observations. I then describe the key limitations of RM-ANOVA: its inability to handle missing data, partially within-participants factors, continuous within-participants covariates, and complex dependency structures such as crossed random effects designs. I also discuss the conceptual and computational challenges associated with LMMs, such as model specification and convergence issues. Using two simulation studies, I then demonstrate that when both methods are appropriate, RM-ANOVA -- especially with Greenhouse-Geisser correction -- generally provides better control of Type I error rates and statistical power equal to or greater than that of LMMs. Consequently, I recommend using RM-ANOVA in designs where RM-ANOVA is appropriate. LMMs should be reserved for cases that require the additional flexibility, such as complex designs, missing data, or advanced modelling goals.