Predictions for the GWB characteristic strain amplitude at f = 10/yr⁻¹ from a range of L-Galaxies semi-analytical model versions, assuming that h_c(f)  ∝  f^−2/3. The error bars are computed taking into account the cosmic variance. To this end, we divided the Millennium box into 125 sub-boxes and we compute the GWB in each one. The standard deviation provided by the 125 GWBs corresponds to the extension of our error bars.