Comparison between the colors of the training and NDWFS-Boötes (photometric and spectroscopic quasars) samples. Black contours and points denote the spectroscopic quasars with z >  1.4 of the training sample, while purple squares indicate all the spectroscopic quasars in Boötes with z >  1.4. Photometric quasars in our sample are denoted by red circles. The training sample is employed to identify quasars in the target sample (see Sect. 3), and to determine their photometric redshifts with the NW kernel regression method (see Sect. 4.1).