been widely used in the literature (e.g., Vestergaard 2002; Greene & Ho 2004), various systematic uncertainties may affect the measurements of both luminosity and velocity. Broad  emission in Type 1 AGNs can add ambiguity in the determination of the optical continuum luminosity at 5100 Å. In lower-luminosity AGNs, dilution by host galaxy starlight can be so severe that it becomes very difficult, if not virtually impossible, to isolate the nonstellar continuum unambiguously. As noted by Wu [et al.]{} (2004), an additional complication arises in sources, such as blazars, whose optical continuum can be subject to contamination by synchrotron emission from jets, which might be partly beamed. Under these circumstances, the observed [$L_{5100}$]{} overestimates the true thermal component that governs the BLR size-luminosity relation.

For nearby AGNs, including the primary reverberation-mapped sample (Kaspi et al. 2000), the velocity dispersion used in the virial expression is usually measured through the width of the broad [H$\beta$]{}line. However, [H$\beta$]{} is at least a factor of 3 weaker than [H$\alpha$]{}, and so from considerations of signal-to-noise ratio (S/N) alone [H$\alpha$]{}, if available, is superior to [H$\beta$]{}. In practice, in some cases [H$\alpha$]{} may be the [*only*]{} detectable broad permitted line in the optical (traditionally such 