Abstract
The symbolic-addition method is known to break down frequently in P{\bar 1} and related space groups. Analyses of the generation of signs in some known cases have been made in order to find useful modifications of the method. Errors can often be traced back to the acceptance of structure factors whose signs have been directly determined from Σ2 relations. When incorrect signs have entered into the data more errors may develop very rapidly. As a consequence, E maps based on a very limited number of the structure factors signed in the first stages may be significantly better than maps calculated in the usual way with all E's above some arbitrary limit. Structural information can be used directly as an aid for selecting the most probable sign models from a given starting set.
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