Histograms of summed residuals below 25 keV obtained from time-integrated simulations of a perfect Band function. The dark line shows the value of the data for that GRB. a) The value of the deviation for GRB 080817A is − 3.6. The mean and the standard deviation of the simulated distribution are − 1.1 and 3.4 respectively. In this case the data does not deviate significantly from the simulated distribution and has a normalised deviation of − 0.7σ. b) The value of the deviation for GRB 090424 is − 25.8 compared to the simulated distribution with a mean and standard deviation of 0.4 and 4.3 respectively. The data deviates significantly from the simulated distribution with a normalised deviation of − 6.1σ.