Varying the item format improved the range of measurement in patient-reported outcome measures assessing physical function.
Physical 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.



Three 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.



Sufficient 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.



Using 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.
Department of Psychosomatic Medicine, Center for Internal Medicine and Dermatology, Charité - Universitätsmedizin Berlin, Charitéplatz 1, 10117, Berlin, Germany. gregor.liegl@charite.de.
Gregor
G
Liegl
Department of Quantitative Health Sciences, University of Massachusetts Medical School, Worcester, MA, USA.|John Ware Research Group, Watertown, MA, USA.
Barbara
B
Gandek
Department of Psychosomatic Medicine, Center for Internal Medicine and Dermatology, Charité - Universitätsmedizin Berlin, Charitéplatz 1, 10117, Berlin, Germany.|Institute for Social Medicine, Epidemiology and Health Economics, Charité - Universitätsmedizin Berlin, Berlin, Germany.
H Felix
HF
Fischer
National Research Centre for the Working Environment, Copenhagen, Denmark.|Optum, Lincoln, RI, USA.|Department of Public Health, University of Copenhagen, Copenhagen, Denmark.
Jakob B
JB
Bjorner
Department of Quantitative Health Sciences, University of Massachusetts Medical School, Worcester, MA, USA.|John Ware Research Group, Watertown, MA, USA.
John E
JE
Ware
Department of Psychosomatic Medicine, Center for Internal Medicine and Dermatology, Charité - Universitätsmedizin Berlin, Charitéplatz 1, 10117, Berlin, Germany.|Department of Quantitative Health Sciences, University of Massachusetts Medical School, Worcester, MA, USA.
Matthias
M
Rose
Department of Immunology and Rheumatology, Stanford University School of Medicine, Palo Alto, CA, USA.
James F
JF
Fries
Department of Psychosomatic Medicine, Center for Internal Medicine and Dermatology, Charité - Universitätsmedizin Berlin, Charitéplatz 1, 10117, Berlin, Germany.|Population Health Strategic Research Centre, School of Health and Social Development, Deakin University, Melbourne, VIC, Australia.
Sandra
S
Nolte
D000328:Adult; D000368:Aged; D004185:Disability Evaluation; D005260:Female; D006801:Humans; D008297:Male; D008875:Middle Aged; D000071066:Patient Reported Outcome Measures; D011594:Psychometrics / Q000379:methods; D012216:Rheumatic Diseases; D011795:Surveys and Questionnaires
Ceiling effects; Item format; Item information; Item-response theory; Measurement range; Patient-reported outcomes; Physical function; Response scale