Results from 10-fold cross-validation on maximum-light spectra. From top to bottom: ℛ only; (x₁,ℛ); (c,ℛ^c); (x₁,c,ℛ^c). The left column is color-coded according to the weighted rms of prediction Hubble residuals, while the right column corresponds to the absolute Pearson cross-correlation coefficient of the correction terms with uncorrected Hubble residuals.