Most of the existing cloud radiation models treat liquid water drops of a variety of sizes as an ensemble of particles. The ensemble approach assumes that all drop sizes are well represented in an elementary volume. A realistic cloud contains a huge number of small drops and a tiny number of large ones; hence, the latter can not be present in every elementary volume. To stay in the framework of the ensemble approach, large drops are artificially fractionated even though this is obviously a poor approximation since drops are discrete. However, classical radiative transfer theory uses this approximation to simplify the radiative transfer process, as if scattering takes place from an "average drop" rather than from a particular drop. In our presentation we analyze the limitations of this approximation in radiative transfer and discuss a possible range of its applicability.