The definition of the indexing figure of merit M20

The interpretation used by B. Khawas [Acta Cryst. (1970), B26, 1919; Acta Cryst. (1971), B27, 1517] of the quantity N20 occurring in the definition of the figure of merit M20 (P. M. de Wolff, J. Appl. Cryst. (1968), 1, 108) for the indexing of powder patterns is not that intended. Calculated Q-values should be counted for N20 whether they explain observed lines or not.