  Results from a few decades of reverberation mapping (RM) studies have
revealed a correlation between the radius of the broad-line emitting region
(BLR) and the continuum luminosity of active galactic nuclei. This
"radius-luminosity" relation enables survey-scale black-hole mass estimates
across cosmic time, using relatively inexpensive single-epoch spectroscopy,
rather than intensive RM time monitoring. However, recent results from newer
reverberation mapping campaigns challenge this widely used paradigm, reporting
quasar BLR sizes that differ significantly from the previously established
radius-luminosity relation. Using simulations of the radius--luminosity
relation with the observational parameters of SDSS-RM, we find that this
difference is not likely due to observational biases. Instead, it appears that
previous RM samples were biased to a subset of quasar properties, and the
broader parameter space occupied by the SDSS-RM quasar sample has a genuinely
wider range of BLR sizes. We examine the correlation between the deviations
from the radius-luminosity relation and several quasar parameters; the most
significant correlations indicate that the deviations depend on UV/optical SED
and the relative amount of ionizing radiation. Our results indicate that
single-epoch black-hole mass estimates that do not account for the diversity of
quasars in the radius-luminosity relation could be overestimated by an average
of ~0.3 dex.
