Studies of well-defined samples of quasars show that the equivalent width of the CIV 1549 emission line for flat-spectrum, radio-loud quasars follow the relation EW(CIV 1549) L(optical)-0.6, but the correlation is much weaker for other broad emission lines and for other categories of AGN.
Figure 1. The equivalent width of the broad CIV 1549 emission line versus luminosity in the adjacent continuum. The crosses are PG quasars with MB < -24 mag; the filled circles are an optical flux-limited subset of flat-spectrum radio quasars from the PKS ± 4 degree survey (Baldwin et al. 1989)
Baldwin, J.A. 1977 ApJ, 214, 679
Baldwin, J.A., Wampler, E.J., & Gaskelll, C.M. 1989, ApJ., 338, 630
Adapted from J. H. Krolik, Active Galactic Nuclei Princeton Series in Astrophysics, 1999