Validation of the EORTC QLQ-C30 quality of life questionnaire through combined qualitative and quantitative assessment of patient-observer agreement

Patient-rated questionnaires are increasingly used to assess health-related quality of life. We studied one aspect of the validity of such measures that has rarely been investigated: do patients interpret ques- tionnaires in the same way as do the researchers reporting the results? If not, there may be a problem. We employed the EORTC QLQ-C30 quality-of-life questionnaire to study 95 cancer patients and measured the agreement between (1) the patient's self-assessment and (2) an observer's rating of the patient's open-ended responses to the same questionnaire administered as an interview. The observer made qualitative recordings describing potential misinterpretations. The agreement between patients' and observers' ratings was high (me- dian kappa = 0.85, range 0.49-1.00). The qualitative data revealed a few minor validity problems. One of these, selective reporting, may lead to systematic errors: some patients reported only what they considered "relevant" symptoms. The combination of quantitative and qualitative methods proved useful for questionnaire validation. J CLIN EPIDEMIOL 50;4:441-450, 1997. 0 1997 Elsevier Science Inc.