Abstract
Patient-reported outcomes are increasingly used as dependent variables in studies regarding the effectiveness of clinical interventions. But patient-reported outcome measures (PROMs) do not provide intuitively meaningful data. For instance, it is not clear what a five point increase or decrease on a particular scale signifies. Establishing ‘interpretability’ involves making changes in outcomes meaningful. Attempts to interpret PROMs have led to the development of methods for identifying a minimum important difference (MID). In this paper, however, I draw on Charles Taylor’s distinction between weak and strong evaluations to suggest that identifying a MID, specifically, a MID that uses a patient-reported reference group, may not provide an adequate interpretation of these measures. Moreover, I argue that the difficulty with interpreting these measures is tied to a larger problem concerning their validity. If researchers wish to interpret PROMs, they may first need to know more about the constructs they attempt to measure, namely, quality of life.
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Notes
To be sure, one might understand this respondent’s answer differently. For instance, as Weinfurt et al. [16] have argued, in the context of informing patients about the risks of Phase I Clinical Trials, there is the possibility of multiple speech acts in any speech episode. The respondent above may intend to convey something different than her words seem to suggest. For instance, she may not aim to provide an evaluation of the place that worry has in a good life, but may instead aim to exude confidence in her radiotherapy or reinforce a positive outlook on her treatment.
In the glossary of the Food and Drug Administration’s 2009 Guidance for Industry Patient-Reported Outcome Measures: Use in Medical Product Development to Support Labeling Claims [21], the FDA defines quality of life as a “general concept that implies an evaluation on the effect of all aspects of life on general well-being.” It goes on to state that because this term involves the evaluation of non-health related aspects of life, and because the term is generally taken to mean what the patient thinks it means, quality of life is too general and undefined to be used for a medical product claim. Part of my argument in this paper, however, is that non-health related aspects of life do impinge on subjective evaluations of health and that these evaluations involve assessments about quality. Moreover, many PROMs do aim to assess quality of life, e.g., European Organization for Research and Treatment of Cancer, Quality of Life Questionnaire, EuroQol, Asthma Quality of Life Questionnaire. Furthermore, many other PROMs include constructs that are associated with quality of life, e.g., social and emotional functioning. These instruments are commonly referred to as ‘Quality of Life (QoL) instruments’; see, e.g., [6].
The phenomenon of response shift, which I discussed earlier, is sometimes taken to contaminate respondent answers. See, e.g., [24].
Many PROMs aim to measure quality of life, but because quality of life is generally taken to be a multidimensional construct, these measures often have multiple scales. Each scale is in turn associated with its own construct, e.g., emotional functioning, mobility, fatigue. The constructs from each scale together provide an assessment of the larger construct, i.e., quality of life.
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McClimans, L. Interpretability, validity, and the minimum important difference. Theor Med Bioeth 32, 389–401 (2011). https://doi.org/10.1007/s11017-011-9186-9
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DOI: https://doi.org/10.1007/s11017-011-9186-9