  Comparison of six high-redshift quasar spectra obtained with the Large
Binocular Telescope with previous observations from the Sloan Digital Sky
Survey shows that failure to correctly identify absorption and other problems
with accurate characterization of the CIV emission line profile in low S/N data
can severely limit the reliability of single-epoch mass estimates based on the
CIV emission line. We combine the analysis of these new high-quality data with
a reanalysis of three other samples based on high S/N spectra of the CIV
emission line region. We find that a large scatter between the Hbeta- and
CIV-based masses remains even for this high S/N sample when using the FWHM to
characterize the BLR velocity dispersion and the standard virial assumption to
calculate the mass. However, we demonstrate that using high-quality data and
the line dispersion to characterize the CIV line width leads to a high level of
consistency between CIV- and Hbeta-based masses, with <0.3 dex of observed
scatter, and an estimated ~0.2 dex intrinsic scatter, in the mass residuals.
