Comparison of simulated GRB populations. Left panel. E_pi as a function of E_iso. Right panel. Slope of the E_pi – E_iso relation as a function of E_iso. Green: Fermi GRBs with no redshift not localized by Swift/BAT; purple: Fermi GRBs with no redshift localized by Swift/BAT; blue: Fermi GRBs with a redshift localized by Swift/BAT; black: Fermi GRBs with a redshift, irrespective of the satellite used for the localization. The crosses show the median values and the corresponding error, measured from 1000 simulated samples. The point symbols show the values measured for two actual samples: in blue 34 GRBs studied in Sect. 5.2, and in black the full sample of 43 GRBs with a redshift.