Normal and half-normal probability plots have been used extensively in the analysis of data and parameters in small-molecule crystallography. A procedure and computer program is described to apply this method to macromolecular data. The utility of this procedure is that a subset of data from a putative heavy-atom derivative can be analyzed and compared with a native set of data providing a quantitative indicator of individual and overall changes in intensity. A qualitative measure of the scattering contribution as a function of resolution can be obtained from comparisons of different resolution ranges. The results from comparisons of (i) native data collected by different techniques and (ii) native and heavy-atom-derivative data suggest a set of guidelines which can be used as an aid in the selection of data with a significant heavy-atom contribution.