Fig. 2

Tree-growing (left) and conditional permutation (CPI; right) predictor space partitioning. In the tree-growing partitioning (left), only one split completely splits the predictor space, all the other splits are conditional on the previous split(s). In the partitioning that is used for permutation scheme in the CPI, where the predictor of interest is X_k=X₁ and \(Z_{(-k)}^{(s)} = X_{2}\) (right), the splits with respect to X₂ are extended to completely split the predictor space (dashed lines) while the splits with respect to X₁ are ignored (gray lines)