Estimated bulk Lorentz factor for the sample of burst with known E_{γ,   iso}. In the top panels, we assume a value of the dimensionless ξ parameter typical of ejecta in the so-called thin shell regime. In the bottom panels, we consider ξ = 0.35, a typical value for thick shells. In the upper (lower) subpanels, we assume an external medium particle density of n_ext = 10   cm^-3 (n_ext = 1   cm^-3). Red diamonds, blue circles, and green triangles correspond to short, intermediate, and long events, respectively. Only the bursts represented with bigger filled symbols possess a Lorentz factor in the allowed range  [ Γ_min,Γ_max ]  (see text).