Feature importance analysis of the combined algorithm redshift estimator based on a modified version of the permutation feature importance technique (see text for details). The y-axis shows the feature importance for the −σ_NMAD scorer. Different colours show the score for the entire sample (black), high-z quasars (orange), and low-z quasars (blue). These should be compared to the measured σ_NMAD = 0.11, 0.03 and 0.19, respectively.