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Comparing translations of the EORTC QLQ-C30 using differential item functioning analyses.
N W Scott1, P M Fayers, A Bottomley
1Department of Public Health, University of Aberdeen, Polwarth Building, Foresterhill, Aberdeen, AB25 2ZD, UK, and Department of Palliative Medicine, Bispebjerg Hospital, Copenhagen, Denmark.
Summary
Linguistic analysis of the European Organisation for Research and Treatment of Cancer (EORTC) QLQ-C30 revealed significant differences in how international patients answered the quality of life questionnaire, impacting clinical trial results.
Area of Science:
- Oncology
- Psychometrics
- Health Services Research
Background:
- The European Organisation for Research and Treatment of Cancer (EORTC) Quality of Life Questionnaire Core 36 (QLQ-C30) is a vital tool for assessing cancer patient quality of life.
- Ensuring linguistic equivalence across different language versions is crucial for accurate cross-cultural comparisons.
Purpose of the Study:
- To investigate potential linguistic variations in responses to the EORTC QLQ-C30 among an international patient sample.
- To identify specific questionnaire items exhibiting differential item functioning (DIF) across different languages.
Main Methods:
- Analysis of a large dataset (27,891 respondents from 103 studies) comprising 13 translations of the EORTC QLQ-C30 across 22 countries.
- Application of logistic regression for differential item functioning (DIF) analysis to detect items answered differently based on language, controlling for subscale scores.
- Assessment of both uniform and non-uniform DIF, supplemented by qualitative interviews with bilingual individuals.
Main Results:
- While most language versions showed results comparable to English, statistically significant DIF was detected for every translation.
- Several scales demonstrated substantial discrepancies in results for specific translations, with some DIF effects large enough to influence clinical trial outcomes.
- Qualitative interviews corroborated some of the observed linguistic patterns.
Conclusions:
- Overall linguistic equivalence is good for most EORTC QLQ-C30 items, but significant variations exist for certain translations.
- The identified linguistic differences can impact the interpretation of results in clinical trials.
- Validation of health-related quality of life instruments should routinely incorporate DIF analyses to ensure cross-cultural validity.