  If accretion disc emission results from turbulent dissipation, then
axisymmetric accretion theory must be used as a mean field theory: turbulent
flows are at most axisymmetric only when suitably averaged. Spectral
predictions therefore have an intrinsic imprecision that must be quantified to
interpret the variability exhibited by a source observed at different epochs.
We quantify contributions to the stochastic imprecision that come from
azimuthal and radial averaging and show that the imprecision is minimized for a
particular choice of radial averaging, which in turn, corresponds to an optimal
spectral resolution of a telescope for a spatially unresolved source. If the
optimal spectral resolution is less than that of the telescope then the data
can be binned to compare to the theoretical prediction of minimum imprecision.
Little stochastic variability is predicted at radii much larger than that at
which the dominant eddy turnover time ($\sim$ orbit time) exceeds the time
interval between observations; the epochs would then be sampling the same
member of the stochastic ensemble. We discuss the application of these
principles to protoplanetary discs for which there is presently a paucity of
multi-epoch data but for which such data acquisition projects are underway.
