The functional assessment of chronic illness therapy-fatigue (FACIT-F) is commonly used to assess fatigue across diseases. The degree to which the FACIT-F demonstrates measurement equivalence across disease groups, however, is not known. The purpose of this study was to assess differential item functioning (DIF) of FACIT-F items between patients with cancer and systemic sclerosis (SSc or scleroderma).
Secondary analysis of FACIT-F data from cancer and SSc patients. Confirmatory factor analysis was used to assess the factor structure of the FACIT-F in cancer and SSc patients. The multiple-indicator, multiple-cause model was utilized to assess DIF, comparing responses from cancer and SSc patients.
A unidimensional factor structure for the FACIT-F was demonstrated with the cancer (n = 1141), SSc (n = 1186), and combined samples. Statistically significant, but small-magnitude, DIF was found for four items. Compared to cancer patients with the same level of fatigue, SSc patients had lower scores (more fatigue) for item 2 ( bodily weakness), 7 ( energy), and 8 ( ability to perform daily activities); and higher scores (less fatigue) for item 9 ( need to sleep throughout the day). For the entire scale, SSc patients had 0.47 SD lower FACIT-F latent factor scores (more fatigue) than cancer patients. After correcting for DIF, there was a change of only 0.03 SD in this difference (0.44 SD lower).
Although statistically significant DIF was detected for four FACIT-F items, the magnitude was small and the effect on fatigue latent scores was minimal. Thus, FACIT-F scores can be used equivalently in cancer and SSc.