GRB duration probability distribution. The blue histogram shows the estimated probability distribution of intrinsic duration (defined as T₉₀/(1 + z)) of Swift-detected GRBs with a measured redshift. Red points represent the corresponding distribution for our simulated populations (assuming an observed rate ratio of 13.5 between LGRB and SGRB, i.e. the same rate ratio as in the Swift sample), while the grey points show the distribution for the LGRB synthetic population alone (downward arrows are upper limits that correspond to empty bins). Vertical error bars show the Poisson uncertainty associated to each bin.