Comparison of the variation of the redshift of the space density peak with radio luminosity (steep-spectrum sources, black solid line; the uncertainty range arising from the assumed spectral index is indicated by dotted lines) and B-band luminosity (quasars, blue dashed line). The subplot shows the ratio between the two luminosities as a function of z_peak. The dotted line here indicates a constant ratio of log (P_{1.4 GHz}/P_{B − band}) = 1.5, the minimum value historically adopted when classifying a source as radio-loud (Kellermann et al. 1989).