The method for the lm class (or for the lvmisc_cv
class of a lm) returns a data frame with the columns AIC
(Akaike information criterion), BIC (Bayesian information
criterion), R2 (R squared), R2_adj (adjusted R squared),
MAE (mean absolute error), MAPE (mean absolute percent
error) and RMSE (root mean square error).
The method for the lmerMod (or for the lvmisc_cv class of a
lmerMod) returns a data frame with the columns R2_marg and
R2_cond instead of the columns R2 and R2_adj.
All the other columns are the same as the method for lm.
R2_marg is the marginal R squared, which considers only the variance
by the fixed effects of a mixed model, and R2_cond is the
conditional R squared, which considers both fixed and random effects
variance.