Extension of Nakagawa & Schielzeth's R²_GLMM to random slopes models

Nakagawa & Schielzeth extended the widely used goodness-of-fit statistic R² to apply to generalized linear mixed models (GLMMs). However, their R²_GLMM method is restricted to models with the simplest random effects structure, known as random intercepts models. It is not applicable to another common random effects structure, random slopes models.I show that R²_GLMM can be extended to random slopes models using a simple formula that is straightforward to implement in statistical software. This extension substantially widens the potential application of R²_GLMM.