Title

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

UMMS Affiliation

Department of Quantitative Health Sciences

Date

8-2015

Document Type

Article

Disciplines

Biostatistics | Epidemiology | Health Services Research | Public Health | Sociology

Abstract

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.

Rights and Permissions

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

Related Resources

Link to Article in PubMed

Keywords

UMCCTS funding

PubMed ID

25601166