lme4 (R package)

lme4 is an R package used for analyzing complex data that involve multiple levels of grouping, such as measurements from students within schools or patients within hospitals. It fits linear mixed-effects models, which account for both fixed effects (consistent factors like treatment) and random effects (variations across groups). This allows researchers to understand how different factors influence outcomes while considering variability at different levels. lme4 is widely used in fields like ecology, psychology, and medicine for modeling hierarchical or clustered data accurately.