The procedure of obtaining the concentration independent partial interference functions Iij(k) which was applied in many articles for the analysis of diffraction experiments on liquid metall alloys is reproduced for the model system of hard spheres. As in the real alloys the functions Iij(k) allow one to reconstruct the total interference functions of the system in the wide range of compositions. However, Iij(k) differ in an essential way from the true partial functions Iij(k) of the model system. Thus, although for every real binary alloy it is possible to find (in a variety of ways) the concentration independent functions Iij(k), they have no clear physical interpretation and, in any case, do not represent the true partial interference functions of the system