Comparison of the distributions of bulk Lorentz factors of GRBs obtained from independent methods. The blue-filled histogram represents the distribution of Γ values derived in this work from the pair-production opacity argument applied to the prompt spectra of 9 GRBs showing a cutoff. The empty histograms show the distributions of Γ values derived from the measurement of the afterglow onset time for a sample of 50 GRBs, as reported in Ghirlanda et al. (2018). The orange histogram represents the values of the bulk Lorentz factor derived assuming a constant medium density, while the green histogram refers to the wind-like medium case. These distributions are built taking into account measurements only (not lower limits), for both methods.