Psychometric evaluation of a questionnaire to document side-effects of chemotherapy

This paper reports the psychometric testing of the Worthing Chemotherapy questionnaire (WCQ). The WCQ is a patient self-report instrument to document side-effects of chemotherapy. Literature review of relevant studies shows that psychometric testing of similar instruments is rarely rigorous. Content validity for the WCQ was established in five ways: literature review, Delphi review among oncology staff, pre-pilot unstructured interviews, pilot study and amendment of the instrument and items for spontaneous reporting of problems on the questionnaire. A three-stage approach to construct validity was used. The hypothesis adopted was that as certain cytotoxic agents cause stomatitis, incidence and severity of stomatitis will decrease following cessation of treatment. Stage 1: factor analysis confirmed the presence of a sole factor, with an eigenvalue of 5.3, for mouth problems which explained 65.5% of the variance. Stage 2: the hypothesis was confirmed using research findings. Stage 3: the Wilcoxon test showed highly significant results for during and post chemotherapy stomatitis scores. Reliability of the questionnaire was assessed using the test-retest method. Weighted kappa was chosen as the test statistic. A median value of (w)k=0.87 was obtained. The results indicate that the WCQ is a reliable and valid instrument.