30-item General Health Questionnaire in general hospitals: selecting items using a stepwise hierarchical procedure.

An attempt was made to improve the validity of the 30-item General Health Questionnaire by excluding items. This exclusion was performed by using correlations of sum-scores with an external case criterion. This hierarchical approach based on subsets of items (stepwise hierarchical variable selection) resulted in a 9-item questionnaire whose discriminating performance was significantly better than that of the original version.