Accounting for Cosmic Variance in Studies of Gravitationally-Lensed
  High-Redshift Galaxies in the Hubble Frontier Field Clusters
  Strong gravitational lensing provides a powerful means for studying faint
galaxies in the distant universe. By magnifying the apparent brightness of
background sources, massive clusters enable the detection of galaxies fainter
than the usual sensitivity limit for blank fields. However, this gain in
effective sensitivity comes at the cost of a reduced survey volume and, in this
{\it Letter}, we demonstrate there is an associated increase in the cosmic
variance uncertainty. As an example, we show that the cosmic variance
uncertainty of the high redshift population viewed through the Hubble Space
Telescope Frontier Field cluster Abell 2744 increases from ~35% at redshift z~7
to >~65% at z~10. Previous studies of high redshift galaxies identified in the
Frontier Fields have underestimated the cosmic variance uncertainty that will
affect the ultimate constraints on both the faint end slope of the
high-redshift luminosity function and the cosmic star formation rate density,
key goals of the Frontier Field program.
