  The derivation of entropy for cluster methods is reformulated by constructing
the probability of a given particle (spin) configuration as a self-consistent
product of cluster configuration probabilities. This approach gives an insight
into the nature of underlying approximations involved at different levels of
the cluster-variation method. The graphical representation of the product
allows us to extend this method for non-Bravais lattices as it is demonstrated
on interstitial sites of body-centered- and face-centered-cubic crystals.
