Objective The reliability and validity of the European Organization for Research and Treatment of Cancer Quality of Life Questionnaire (EORTC QLQ-C30) has not been examined while taking into account the correlation between subscales. The reliability of the EORTC QLQ-C30 subscales is modest, thus limiting their utility in both clinical and research settings. The purpose of this study was to validate the factor structure of multiple-item subscales of the EORTC QLQ-C30 and to improve their reliability by means of an item response analysis by using the multidimensional partial credit model. Methods A total of 2295 patients with complete data were used for the analysis. One- and nine-dimensional partial credit models were used to fit the data to validate the construct validity of the multiple-item subscales of the QLQ-C30. Results The model comparison showed that the nine-dimensional factor structure of multiple-item subscales was satisfactory. The multidimensional partial credit model fit data of the multiple-item subscales of the QLQ-C30 reasonably well. The estimated test reliabilities of each domain obtained from the multidimensional approach were higher than those obtained from the unidimensional approach. Conclusions The constructs represented by the multiple-item subscales of the QLQ-C30 were validated. The improved reliability of the multiple-item subscales of the QLQ-C30 under the multidimensional approach can facilitate their applications in clinical and research settings.
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