A modified least-squares analysis is presented that allows reliable structural parameters to be extracted from a powder diffraction pattern even in the presence of a substantial unmodelled impurity contribution. The algorithm is developed within the context of Bayesian probability theory. Experimental points that fall above those calculated, and are thus more probably from impurity peaks, are systematically down-weighted. This approach is illustrated with a two-phase example. © 2001 International Union of Crystallography Printed in Great Britain - all rights reserved.
