  Cosmic variance is the uncertainty in observational estimates of the volume
density of extragalactic objects such as galaxies or quasars arising from the
underlying large-scale density fluctuations. This is often a significant source
of uncertainty, especially in deep galaxy surveys, which tend to cover
relatively small areas. We present estimates of the relative cosmic variance
for one-point statistics (i.e. number densities) for typical scales and volumes
sampled by the Great Observatories Origins Deep Survey (GOODS). We use two
approaches: for objects with a known two-point correlation function that is
well-approximated by a power law, one can use the standard analytic formalism
to calculate the cosmic variance (in excess of shot noise). We use this
approach to estimate the cosmic variance for several populations that are being
studied in the GOODS program: Extremely Red Objects (ERO) at z~1, and Lyman
Break Galaxies (LBG) at z~3 and z~4, using clustering information for similar
populations in the literature. For populations with unknown clustering, one can
use predictions from Cold Dark Matter theory to obtain a rough estimate of the
variance as a function of number density. We present a convenient plot which
allows one to use this approach to read off the cosmic variance for a
population with a known mean redshift and estimated number density. We conclude
that for the volumes sampled by GOODS, cosmic variance is a significant source
of uncertainty for strongly clustered objects (~40-60% for EROs) and less
serious for less clustered objects, ~10-20% for LBGs.
