  We evaluate the effect of structure formation on the average expansion rate
with a statistical treatment where density peaks and troughs are modelled as
homogeneous ellipsoids. This extends earlier work that used spherical regions.
We find that the shear and the presence of filamentary and planar structures
have only a small impact on the results. The expansion rate times the age of
the universe $Ht$ increases from 2/3 to 0.83 at late times, in order of
magnitude agreement with observations, although the change is slower and takes
longer than in the real universe. We discuss shortcomings that have to be
addressed for this and similar statistical models in the literature to develop
into realistic quantitative treatment of backreaction.
