In this paper, some basic concepts on effective properties of nonhomogeneous elastic solids are reviewed. The various theories are evaluated by conducting experiments on artificially cracked and porous solids, and by comparing the results with the theoretical predictions for the cases of interacting and non-interacting inhomogeneities. Two aluminum plates containing slots and two containing circular holes in random mutual positions with different orientational distributions were tested. In the case of plates containing random slots it is shown that the approximation of non-interacting cracks provides a reasonable estimate of the modulus, even when interactions may be significant. The results obtained from the plates containing circular holes indicate that, as porosity increases, the effective Young's modulus follows the predictions for the case of interacting holes from both Mori-Tanaka and differential schemes.