  It is well known that reverberation mapping of active galactic nuclei (AGN)
reveals a relationship between AGN luminosity and the size of the broad-line
region, and that use of this relationship, combined with the Doppler width of
the broad emission line, enables an estimate of the mass of the black hole at
the center of the active nucleus based on a single spectrum. An unresolved key
issue is the choice of parameter used to characterize the line width, either
FWHM or line dispersion (the square root of the second moment of the line
profile). We argue here that use of FWHM introduces a bias, stretching the mass
scale such that high masses are overestimated and low masses are
underestimated. Here we investigate estimation of black hole masses in AGNs
based on individual or "single epoch" observations, with a particular emphasis
in comparing mass estimates based on line dispersion and FWHM. We confirm the
recent findings that, in addition to luminosity and line width, a third
parameter is required to obtain accurate masses and that parameter seems to be
Eddington ratio. We present simplified empirical formulae for estimating black
hole masses from the Hbeta 4861 A and C IV 1549 A emission lines. While the AGN
continuum luminosity at 5100 A is usually used to predict the Hbeta
reverberation lag, we show that the luminosity of the Hbeta broad component can
be used instead without any loss of precision, thus eliminating the difficulty
of accurately accounting for the host-galaxy contribution to the observed
luminosity.
