The difference d_FOM between the FoM_TOT for a given prior value σ_prior and the fiducial FoM_TOT, obtained for the least stringent, default values of the different priors we apply. Upper panel: effect of tightening the priors on the nuisance parameters in the bias terms for ϵϵ (dashed line), nn (dotted line), and all (solid) correlations. Lower panel: effect of tightening the priors on the slopes of the luminosity function α⁽ⁱ⁾, and for the prior on the uncertainty of σ_ph. Grey curves correspond to priors on α⁽ⁱ⁾, black curves to those on σ_ph. Except for the ϵϵ case the latter remain very close to zero. As above, ϵϵ correlations are shown as dashed lines, nn correlations as dotted lines, and all correlations as solid lines. Note that ϵϵ correlations do not depend on the α⁽ⁱ⁾. For all curves, the remaining priors are each set to their default values of 50 for the bias term parameters, and 10 for priors on both α⁽ⁱ⁾ and σ_ph.