Evolution of $T_{90}^{\mathrm{src}}$ at each z_sim divided by the corresponding value at z_meas, for all GRBs in the sample. The curves are colour-coded according to the subset to which the GRBs belong. The black horizontal line shows the value used to categorise the GRBs according to their T₉₀ evolution with redshift. The left panel contains all GRBs in the sample, while the right panel contains only those in the lowest-z and highest-z subsets.