Background: The European Organization for research and Treatment of Cancer (EORTC) Core Quality of Life Questionnaire (QLQ-C30) scales are scored on a 4-point response scale, ranging from not at all to very much. Previous studies have shown that the German translation of the response option quite a bit as mäßig violates interval scale assumptions, and that ziemlich is a more appropriate translation. The present studies investigated differences between the two questionnaire versions and were based on the hypothesis that the conventional version yielded lower functioning and higher symptom ratings than the optimized version, particularly in respondents with a higher health burden.
Methods: The first study employed a balanced cross-over design and included 450 patients with different types of cancer from three German-speaking countries. The second study was a representative survey in Germany including 2033 respondents. Half of the participants filled in the mäßig, the other half the ziemlich version of the questionnaire.
Results: In accordance with our hypothesis, the adjusted summary score was lower in the mäßig than in the ziemlich version; Study 1: -4.5 (95% CI -7.8 to -1.3), p = 0.006, Study 2: -3.1 (95% CI - 4.6 to -1.5), p < 0.001. In both studies, this effect was pronounced in respondents with a higher health burden; Study 1: -6.8 (95% CI -12.2 to -1.4), p = 0.013; Study 2: -4.5 (95% CI -7.3 to -1.7), p = 0.002.
Conclusions: We found subtle but consistent differences between the two questionnaire versions. The optimized response option should be used for the EORTC QLQ-C30 as well as for all other German modules.
Trial registration: The study was retrospectively registered on the German Registry for Clinical Studies (reference number DRKS00012759, 04th August 2017, https://www.drks.de/DRKS00012759).