Distributions of precision (black histogram) and recall (blue histogram for the sample of high-z QSOs with z > 2.5, red histogram for the sample with z > 3) as measured on 100 randomly selected test datasets (Sect. 5.6). The upper panel shows the results obtained with the direct selection method (Sect. 5.3) while the lower panel shows the same quantities obtained with the reverse selection method (Sect. 5.5).