Ethnic differential item functioning in the assessment of quality of life in cancer patients

Item response theory methods were used to assess differential item functioning (DIF) in the quality of life (measured by the EORTC QLQ-C30) of cancer patients across four ethnic groups (Caucasian, Filipino, Hawaiian, and Japanese). The sample consisted of 359 cancer patients.Results showed the presence of DIF on several items, indicating ethnic differences in the assessment of quality of life. Relative to the Caucasian and Japanese groups, items related to physical functioning, cognitive functioning, social functioning, nausea and vomiting, and financial difficulties exhibited DIF for Filipinos. On these items Filipinos exhibited either higher or lower QoL scores, even though their overall QoL was the same.This evidence may explain why Filipinos have previously been found to have lower overall QoL. Although Filipinos score lower on QoL than other groups, this may not reflect lower QoL, but rather differences in how QoL is defined. The presence of DIF did not appear, however, to alter the psychometric properties of the QLQ-C30.In recent years medical researchers have shown increasing interest in the physical, psychological, and social health of individuals suffering from disease and treatment-related toxicity [1-3]. These broad characteristics are generally grouped under the inclusive heading quality of life (QoL), and offer a contrast to the more traditional biomedical markers, such as survival time or disease remission. A general definition of QoL is patients' perspectives on their ability to live useful and fulfilling lives, as influenced by, but not completely dependent on disease and treatment [1]. As an instrument of measurement in the clinical setting, QoL is defined functionally by patients' own perceptions of their performance in physical, occupational, psychological, social, financial, and somatic (i.e., physical symptomatology) areas [4,5].The QoL construct is an important one with respect to measuring disease progress and treatment effectiveness. Because