Characterization of the sample. Upper panel: sky map with the positions of the sources studied in this work, which include 237 FSRQs, 88 BL Lacs, three narrow-line Seyferts, and three objects classified as BCUs in the Fermi catalogs. Lower panels: distribution of FSRQs (left) and BL Lacs (right) in the sample as a function of redshift and gamma-ray luminosity (dark points). For comparison, we show as lighter points to the distribution of the respective overall population, based on the luminosity functions by Ajello et al. (2012, 2014). The gray area marks the subthreshold region for the Fermi-LAT. The black distributions alongside the margins refer to the sample; the green (left) and purple (right) distributions refer to the population of sources detected by the Fermi-LAT (i.e., excluding the putative sources below the sensitivity threshold).