Averaging Inhomogeneous Universes: Volume, Angle, Line of Sight
  Cosmologies that match in a volume averaged sense need not generally have the same light propagation behaviors. In particular a universe with inhomogeneity may not demonstrate the Friedmann-Robertson-Walker distance-redshift relation even after volume averaging to FRW spacetime. Even the Dyer-Roeder prescription for incorporating inhomogeneity within a universe equivalent to FRW in an angle averaged sense does not guarantee FRW behavior in general. To legitimately use the FRW distance-redshift relation to interpret observations, the physical conditions must match in a line of sight sense (defined herein: most observations do), since light probes the mass distribution or geometry on all length scales. 
Eric V. Linder (UMass - Amherst)