  Upcoming deep galaxy surveys with JWST will probe galaxy evolution during the
epoch of reionisation (EoR, $5\leq z\leq10$) over relatively compact areas
(e.g. $\sim$ 300\,arcmin$^2$ for the JADES GTO survey). It is therefore
imperative that we understand the degree of survey variance, to evaluate how
representative the galaxy populations in these studies will be. We use the
First Light And Reionisation Epoch Simulations (FLARES) to measure the galaxy
bias of various tracers over an unprecedentedly large range in overdensity for
a hydrodynamic simulation, and use these relations to assess the impact of bias
and clustering on survey variance in the EoR. Star formation is highly biased
relative to the underlying dark matter distribution, with the mean ratio of the
stellar to dark matter density varying by a factor of 100 between regions of
low and high matter overdensity (smoothed on a scale of 14$\,h^{-1}$cMpc). This
is reflected in the galaxy distribution --the most massive galaxies are found
solely in regions of high overdensity. As a consequence of the above, galaxies
in the EoR are highly clustered, which can lead to large variance in survey
number counts. For mean number counts $N\lesssim 100$ (1000), in a unit
redshift slice of angular area 300\,arcmin$^2$ (1.4\,deg$^2$), the 2-sigma
range in $N$ is roughly a factor of four (two). We present relations between
the expected variance and survey area for different survey geometries; these
relations will be of use to observers wishing to understand the impact of
survey variance on their results.
