Improved figures of merit for direct methods

The figures of merit (FOMs) proposed in a recent paper [Cascarano, Giacovazzo & Viterbo (1987). Acta Cryst. A43, 22–29] are modified to improve their efficiency. It is shown that FOMs based on psi-zero triplets and FOMs based on the distribution of the concentration parameter α can be treated by the same mathematical formalism. An earlier FOM, the R_Karle criterion, is revised and suggestions are given for its efficient use. The use of the correlation coefficient is also suggested, provided large- and small-intensity reflections are used simultaneously.