Example histograms of density increments of random apertures and FRB sight lines. Panels (a) and (b) show examples of FRBs in high and low galaxy-number-density environments, respectively, calculated by using WISE × SCOS samples. In each panel, the blue histogram demonstrates the density increment of random apertures, and the vertical red line shows the density increment of the FRB sight line. The ±1σ errors are shown by the red shaded region.