Abstract
BackgroundPhysical function (PF) is a core patient-reported outcome domain in clinical trials in rheumatic diseases. Frequently used PF measures have ceiling effects, leading to large sample size requirements and low sensitivity to change. In most of these instruments, the response category that indicates the highest PF level is the statement that one is able to perform a given physical activity without any limitations or difficulty. This study investigates whether using an item format with an extended response scale, allowing respondents to state that the performance of an activity is easy or very easy, increases the range of precise measurement of self-reported PF.MethodsThree five-item PF short forms were constructed from the Patient-Reported Outcomes Measurement Information System (PROMIS®) wave 1 data. All forms included the same physical activities but varied in item stem and response scale: format A (“Are you able to …”; “without any difficulty”/“unable to do”); format B (“Does your health now limit you …”; “not at all”/“cannot do”); format C (“How difficult is it for you to …”; “very easy”/“impossible”). Each short-form item was answered by 2217–2835 subjects. We evaluated unidimensionality and estimated a graded response model for the 15 short-form items and remaining 119 items of the PROMIS PF bank to compare item and test information for the short forms along the PF continuum. We then used simulated data for five groups with different PF levels to illustrate differences in scoring precision between the short forms using different item formats.ResultsSufficient unidimensionality of all short-form items and the original PF item bank was supported. Compared to formats A and B, format C increased the range of reliable measurement by about 0.5 standard deviations on the positive side of the PF continuum of the sample, provided more item information, and was more useful in distinguishing known groups with above-average functioning.ConclusionsUsing an item format with an extended response scale is an efficient option to increase the measurement range of self-reported physical function without changing the content of the measure or affecting the latent construct of the instrument.
Highlights
Physical function (PF) is a core patient-reported outcome domain in clinical trials in rheumatic diseases
The calibration sample had a wide range of PF, with empirically observed T-scores ranging from 11.1 to 73.6
Our findings suggest that it is possible to reduce ceiling effects by optimizing the item format without changing the content of the measures, which may be especially relevant for the future development of items for static PF measures for use in heterogeneous populations with a broad range of ability
Summary
Physical function (PF) is a core patient-reported outcome domain in clinical trials in rheumatic diseases. Used PF measures have ceiling effects, leading to large sample size requirements and low sensitivity to change In most of these instruments, the response category that indicates the highest PF level is the statement that one is able to perform a given physical activity without any limitations or difficulty. Traditional PF instruments with a fixed number of items, such as the 10-item Medical Outcome Study Short Form-36 (MOS SF-36®) Health Survey physical functioning scale (PF-10) [8] and the 20-item Health Assessment Questionnaire Disability Index (HAQ-DI) [9], have to compromise between clinical practicality and measurement precision, leading to a limited measurement range on the continuum of physical ability [10]. The National Institutes of Health (NIH)-funded Patient-Reported Outcomes Measurement Information System (PROMIS®) has been applying this approach for over 10 years, thereby demonstrating the relevance of IRT item calibration
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