  In general, the mechanics of granular matter is described using continuum
mechanics approach; this requires to introduce the concepts of stress and
strain, which are averaged quantities, so that this needs also to introduce the
notion of representative elementary volume (REV) above which averaged
quantities have some physical meaning. As local quantities fluctuate spatially
in granular matter; a local measure of stress and strain shall exhibit
fluctuations too, whose typical amplitude depends on the sampling size L. This
paper discusses this problem and the causes for large scale correlation. The
mean stress s applied to a plane surface of size L*L is calculated and its
fluctuation amplitude Ds is found when local forces are not correlated; it is
found that Ds/s scales as 1/L . It is shown also that large scale fluctuations
of stress can always be interpreted as an inhomogeneous stress field and that
static equilibrium modifies the mean stress applied to a rod (in 2d), even if
it does not perturb the contact force distribution. This last result is
compared to experiment, which indicates that the number N of contacts per rod
(in 2d) is 2<N<3 . Pacs # : 5.40 ; 45.70 ; 62.20 ; 83.70.Fn
