Theoretical expressions for the probability of success of the quasi-anomalous method have been derived for triclinic, monoclinic and orthorhombic crystals containing a few (1, 2 or 3) heavy atoms per asymmetric unit besides a large number of light atoms. The results derived take into account data-truncation due to unobserved reflections. Using the theoretical expressions, tables of probability values for the success of the quasi-anomalous method are obtained as a function of the relevant parametersk andδ₁². Corresponding results for triclinic crystals containing many heavy atoms (i.e.P = MN and MC cases) have also been obtained. It is seen that, using suitable heavy atoms to prepare the heavy-atom derivative, probability of success as high as 0.7 could be obtained in the case of proteins containing 1000 to 1500 atoms.

Theoretical expressions for the overall values of three Booth type R-indices based on intensity variables are derived. The results are applicable to crystals of any space group containing any number and type of atoms at general positions in the asymmetric unit. The theoretical results were tested in the case of models of a few crystal structures.