MAD phasing: probabilistic estimate of |F0a|

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 |F1+|, |F1-|, |F2+|, |F2-| is used to predict the value of |Foa| 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.