Testing item response theory invariance of the standardized Quality-of-life Disease Impact Scale (QDIS) in acute coronary syndrome patients: differential functioning of items and test
Department of Quantitative Health Sciences
Biostatistics | Epidemiology | Health Services Research | Public Health | Sociology
PURPOSE: The Quality-of-life (QOL) Disease Impact Scale (QDIS(R)) standardizes the content and scoring of QOL impact attributed to different diseases using item response theory (IRT). This study examined the IRT invariance of the QDIS-standardized IRT parameters in an independent sample.
METHOD: The differential functioning of items and test (DFIT) of a static short-form (QDIS-7) was examined across two independent sources: patients hospitalized for acute coronary syndrome (ACS) in the TRACE-CORE study (N = 1,544) and chronically ill US adults in the QDIS standardization sample. "ACS-specific" IRT item parameters were calibrated and linearly transformed to compare to "standardized" IRT item parameters. Differences in IRT model-expected item, scale and theta scores were examined. The DFIT results were also compared in a standard logistic regression differential item functioning analysis.
RESULTS: Item parameters estimated in the ACS sample showed lower discrimination parameters than the standardized discrimination parameters, but only small differences were found for thresholds parameters. In DFIT, results on the non-compensatory differential item functioning index (range 0.005-0.074) were all below the threshold of 0.096. Item differences were further canceled out at the scale level. IRT-based theta scores for ACS patients using standardized and ACS-specific item parameters were highly correlated (r = 0.995, root-mean-square difference = 0.09). Using standardized item parameters, ACS patients scored one-half standard deviation higher (indicating greater QOL impact) compared to chronically ill adults in the standardization sample.
CONCLUSION: The study showed sufficient IRT invariance to warrant the use of standardized IRT scoring of QDIS-7 for studies comparing the QOL impact attributed to acute coronary disease and other chronic conditions.
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Citation: Qual Life Res. 2015 Aug;24(8):1809-22. doi: 10.1007/s11136-015-0916-8. Epub 2015 Jan 20. Link to article on publisher's site
Deng, Nina; Anatchkova, Milena D.; Waring, Molly E.; Han, Kyung T.; and Ware, John E. Jr., "Testing item response theory invariance of the standardized Quality-of-life Disease Impact Scale (QDIS) in acute coronary syndrome patients: differential functioning of items and test" (2015). Quantitative Health Sciences Publications and Presentations. 1139.