  Determining the virial factor of the broad-line region (BLR) gas is crucial
for calibrating AGN black hole mass estimators, since the measured
line-of-sight velocity needs to be converted into the intrinsic virial
velocity. The average virial factor has been empirically calibrated based on
the M-sigma relation of quiescent galaxies, but the claimed values differ by a
factor of two in recent studies. We investigate the origin of the difference by
measuring the M-sigma relation using an updated galaxy sample from the
literature, and explore the dependence of the virial factor on various fitting
methods. We find that the discrepancy is primarily caused by the sample
selection, while the difference stemming from the various regression methods is
marginal. However, we generally prefer the FITEXY and Bayesian estimators based
on Monte Carlo simulations for the M-sigma relation. In addition, the choice of
independent variable in the regression leads to ~0.2 dex variation in the
virial factor inferred from the calibration process. Based on the determined
virial factor, we present the updated M-sigma relation of local active
galaxies.
