Objectives: To explore how patient-reported health related quality of life (HRQL) and global health status are affected by use of differing personal reference frames. We hypothesised that implicit comparisons against self at an earlier time, against healthy peers or against ill patients would greatly affect patients’ response values. Study design and setting: Patients in a randomised trial for treatment of Paget’s disease completed annual HRQL questionnaires. Supplementary questions were appended, asking the patients whether they were aware of having made implicit comparisons. Results: The majority of patients reported considering themselves a year ago (31% at baseline), themselves before becoming ill (23%) or other healthy people (24%), with similar proportions during follow up. Mean HRQL scores varied substantially according to the declared frame of reference, with differences as big as 19% of the scale score, or a standardised mean effect size of 0.74 standard deviations. Conclusion: Reported reference frames were associated with effects of similar magnitude to the differences in HRQL that are regarded as clinically important. This may be of particular concern in trials that andomise patients to management in different settings, such as treatment at home / in hospital, or surgery / chemotherapy, and might bias or obscure HRQL differences
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