Our GRB sample in the peak flux–R mag plane. GRBs with a redshift measurement are represented in blue (circles for GRB afterglows with a well-measured R magnitude, downward triangles for optical upper limits). GRBs without a redshift measurement are represented in black (stars for GRB afterglows with a well-measured R magnitude, downward triangle for optical upper limits). The red dashed lines represent the limiting peak flux and the limiting R magnitude for GRB detection and redshift measurement, respectively. The black dashed line divides the population of GRBs that would undergo the γ-ray selection effect before the optical one (above this line) from those that first suffer from the optical selection effect (below this line).