The inclusion-exclusion formula expresses the size of the union of a family of sets in
terms of the sizes of intersections of all subfamilies. N. Linial and N. Nisan use linear
programming to approximate the size of the union when the intersection sizes are known
only for certain subfamilies. In this article we use purely combinatorial methods to
generalize some of their results. As an application we will construct a contrast optimal
-out-of-
visual cryptography scheme.