 There is no theoretical maximum, but in practice, values are considerably less than one even for poor models, provided the model includes a suitable scale factor.Random experimental errors in the data contribute to formula_3 even for a perfect model, and these have more leverage when the data are weak or few, such as for a low-resolution data set.Model inadequacies such as incorrect or missing parts and unmodeled disorder are the other main contributors to formula_3, making it useful to assess the progress and final result of a crystallographic model refinement.