MultiSpline - Spline-Based Nonlinear Modeling for Multilevel and Longitudinal
Data
Provides a unified framework for fitting, predicting, and
interpreting nonlinear relationships in single-level,
multilevel, and longitudinal regression models. Flexible
functional forms are supported using natural cubic splines
('splines'), B-splines ('splines'), and GAM smooths ('mgcv').
Supports two-way and nested clustering via 'lme4', automatic
knot selection by AIC or BIC, multilevel R-squared
decomposition (marginal and conditional R-squared of Nakagawa
and Schielzeth (2013) <doi:10.1111/j.2041-210x.2012.00261.x>,
extended to random slopes as in Johnson (2014)
<doi:10.1111/2041-210X.12225>, with level-specific variance
partitioning), a postestimation suite returning first and
second derivatives with confidence bands, turning points and
inflection regions, and a model comparison workflow contrasting
linear, polynomial, and spline fits by AIC, BIC, and
likelihood-ratio tests. Cluster heterogeneity in nonlinear
effects is supported via random-slope spline terms. Any 'lme4'
random-effects structure can be supplied, and
population-averaged (marginal) curves are available for
generalized linear mixed models.