P(z_s|DM_exc) projections after application of Bayes’ theorem, illustrating potential source redshift z_s ranges for FRBs with observed excess electron components DM_exc. The model chosen assumes FRB progenitors to follow YPSR distributions in spiral galaxies. Left subplot: DM_exc projections (solid lines) against P(z_s|DM_IGM) (dashed lines), illustrating the influence of the host contribution. A uniform (U(0,6) prior redshift distribution is assumed. Right subplot: projections for an FRB of DM_exc equal to thatof FRB 121102. Uniform (pink) and Erlang (P(z_s) = P(z_s;k, γ)) prior redshift distributions are compared. Vertical lines indicating FRB 121102’s redshift as predicted conventionally (dash-dotted) and as observed spectroscopically (dashed) are included. Models are normalised to ensure ∫ P(z_s |DM) dz = 1.