Equating EORTC QLQ-C30 and FACT-G scores and its use in oncological research

Objective: To investigate the equivalence of the European Organization for Research and Treatment of Cancer Core Questionnaire (EORTC QLQ-C30) and the Functional Assessment for Cancer Therapy - General (FACT-G) on the basis of corresponding subscales, and where appropriate to derive a scheme for converting QLQ-C30 scores into FACT-G scores and vice versa for use in oncological research. Method: A calibration sample of 737 cancer patients (mean age 51.4 +/- 7.6 (SD), 63% female, 25% with current chemotherapy) who filled in both quality of life (QOL) questionnaires was used. Both classical test theory and the Rasch measurement model were applied. Results: Three of the four subscales common to both QOL instruments (physical, emotional, functional) proved suitable for equating (acceptable inter-correlations of corresponding subscales physical (r = 0.77), emotional domain (r = 0.60) role/functional (r = 0.63) relative to their internal consistency, sufficient unidimensionality of pooled subscales, satisfactory fit to the Rasch model). Conversion tables for these subscales were generated. Conclusions: The conversion tables developed in this study (physical, emotional and functional/role domain) appear promising for the comparison between EORTC QLQ-C30 and FACT-G scores of patient samples.