  We present robust statistical estimates of the accuracy of early-type galaxy
stellar masses derived from spectral energy distribution (SED) fitting as
functions of various empirical and theoretical assumptions. Using large samples
consisting of 40,000 galaxies from the Sloan Digital Sky Survey, of which 5,000
are also in the UKIRT Infrared Deep Sky Survey, with spectroscopic redshifts in
the range 0.05 \leq z \leq 0.095, we test the reliability of some commonly used
stellar population models and extinction laws for computing stellar masses.
Spectroscopic ages (t), metallicities (Z), and extinctions (A) are also
computed from fits to SDSS spectra using various population models. These
constraints are used in additional tests to estimate the systematic errors in
the stellar masses derived from SED fitting, where t, Z, and A are typically
left as free parameters. We find reasonable agreement in mass estimates among
stellar population models, with variation of the IMF and extinction law
yielding systematic biases on the mass of nearly a factor of 2, in agreement
with other studies. Removing the near-infrared bands changes the statistical
bias in mass by only 0.06 dex, adding uncertainties of 0.1 dex at the 95% CL.
In contrast, we find that removing an ultraviolet band is more critical,
introducing 2{\sigma} uncertainties of 0.15 dex. Finally, we find that stellar
masses are less affected by absence of metallicity and/or dust extinction
knowledge. However, there is a definite systematic offset in the mass estimate
when the stellar population age is unknown, up to a factor of 2.5 for very old
(12 Gyr) stellar populations. We present the stellar masses for our sample,
corrected for the measured systematic biases due to photometrically determined
ages, finding that age errors produce lower stellar masses by 0.15 dex, with
errors of 0.02 dex at the 95% CL for the median stellar age subsample.
