Precision (dashed black line), recall at z > 2.5 (blue line), and at z > 3 (red line) as a function of the XGBoost num_round parameter for the direct selection method (upper panel), direct selection with under-sampling (middle panel), and reverse selection (lower panel) methods. The precision curve in the middle panel is multiplied by a factor 100 to provide a clearer view. The point and error bars represent respectively the mean and the standard deviation over five runs with randomly chosen training-test splits.