Feature importance analysis of the combined algorithm classifier based on a modified version of the permutation feature importance technique (see text for details). The y-axis shows the feature importance for different scorers (f₁ score, purity, and completeness), and for high-z and low-z quasars separately. These should be compared with the performance values listed in Table 3. Columns rSDSS, QPz, and LePhare are not included in our final classifier.