Excess electron models P(DM_exc|z_s) illustrating the relationship between observed excess dispersion measure DM_exc and source redshift z_s for simulated FRB populations. Subplots consider progenitor scenarios following: (a) YPSR distributions in spiral galaxies, homogeneous distributions within galactocentric spheres of radius; (b) H = 0.2 kpc; (c) H = 0.004 kpc in spiral galaxies; elliptical distributions confined to regions (d) 1; (e) 0.1 times the effective radius (R_e = 3.2 kpc) of elliptical galaxies; and (f) homogeneous distributions within galactocentric spheres of radius H = 0.1 kpc in elliptical galaxies. Projections of each model for different z_s (solid lines) are compared with P(DM_IGM|z_s) (dotted lines) to illustrate the influence of the host contribution. The two-dimensional PDFs themselves (z_s against DM_exc with colour indicating relative likelihood) are inset into each subplot. Models are normalised to ensure ∫ P(DM_exc|z_s) dDM = 1.