Patients and methods: Between April 2010 and April 2012, a total of 300 patients with RA (77 males, 223 females; mean age: 52.3±11.5 years; range, 18 to 82 years) who answered items from a range of widely used instruments were included. After examining initial dimensionality with exploratory factor analysis (EFA), confirmatory factor analysis (CFA), and Rasch analysis were used to evaluate the tools` construct validities. The data-model fit was evaluated with goodness-of-fit (GoF) statistics in CFA, while the tools were examined in terms of item and person fit, unidimensionality and differential item functioning (DIF) from the perspective of Rasch analysis.

Results: According to EFA, two dimensions were identified and named as `self-care-mobility-household activities related to lower extremity` and `self-care-mobility-household activities related to upper extremity` taking into account the factor loadings and the clinical classifications. While the clinical classification was tested with CFA, all items were loaded on their pre-defined dimensions with the factor loadings of ≥0.40 and GoF statistics were within the acceptable ranges. When the `self-care-mobility-household activities related to upper extremity` and `self-care-mobility-household activities related to lower extremity` tools were evaluated via the Rasch analysis, both tools were found to fit the Rasch model expectations, with a mean item fit statistics of -0.528 logit (standard deviation [SD]: 1.365) and -0.213 (SD: 1.168; mean person fit statistics of -0.412 logit (SD: 1.160) and -0.303 logit (SD: 0.859), respectively.

Conclusion: For the evaluation of a scale`s construct validity, it is recommended to use the Rasch analysis in tandem with factor analytic methods, as the Rasch analysis explores a scale`s construct validity in terms of item and person fit, DIF and unidimensionality which is the only aspect of the factor analysis. Keywords : Factor analysis, functional status, measurement tool, Rasch analysis, rheumatoid arthritis