MAD phasing: probabilistic estimate of [F (oa)].

The method of the joint probability distribution function is applied in order to estimate the structure-factor moduli of the anomalous scatterer substructure. The two-wavelength case is examined: the prior knowledge of the moduli [F(1)(+)], [F(1)(-)], [F(2)(+)], [F(2)(-)] is used to predict the value of [F(oa)] arising from the normal scattering of the anomalous scatterers. The conclusive formula is applied to ideal and to real cases: evidence of the usefulness of the approach is obtained.