  We prove global well-posedness for the defocusing cubic wave equation with
data in $H^{s} \times H^{s-1}$, $1>s>{13/18}$. The main task is to estimate the
variation of an almost conserved quantity on an arbitrary long time interval.
We divide it into subintervals. On each of these subintervals we write the
solution as the sum of its linear part adapted to the subinterval and its
corresponding npnlinear part. Some terms resulting from this decomposition have
a controlled global variation and other terms have a slow local variation.
