About the sigma-A estimate

The resolution parameter Sigma (A) is currently used for evaluating the degree of similarity between a model and the target structure. Here, quasi-Wilson distributions are used to represent the local statistics of the normalized amplitudes both for the target and for the model structure. The study uses the joint probability distribution approach to provide (i) a description of the statistical properties of the Sigma (A) parameter; (ii) a deeper insight into the role, for the Sigma (A) estimate, of the high-order moments of the target and of the model structure-factor distributions; and (iii) new statistical formulas for estimating Sigma (A) . The theoretical results have been checked using test proteins.