  Black hole mass is a fundamental property of active galactic nuclei (AGNs).
In the distant universe, black hole mass is commonly estimated using the MgII,
Hbeta, or Halpha emission line widths and the optical/UV continuum or line
luminosities, as proxies for the characteristic velocity and size of the
broad-line region. Although they all have a common calibration in the local
universe, a number of different recipes are currently used in the literature.
It is important to verify the relative accuracy and consistency of the recipes,
as systematic changes could mimic evolutionary trends when comparing various
samples. At z=0.36, all three lines can be observed at optical wavelengths,
providing a unique opportunity to compare different empirical recipes. We use
spectra from the Keck Telescope and the Sloan Digital Sky Survey to compare
black hole mass estimators for a sample of nineteen AGNs at this redshift. We
compare popular recipes available from the literature, finding that mass
estimates can differ up to 0.38+-0.05 dex in the mean (or 0.13+-0.05 dex, if
the same virial coefficient is adopted). Finally, we provide a set of 30
internally self consistent recipes for determining black hole mass from a
variety of observables. The intrinsic scatter between cross-calibrated recipes
is in the range 0.1-0.3 dex. This should be considered as a lower limit to the
uncertainty of the black hole mass estimators.
