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The Brief Illness Perception Questionnaire (B-IPQ) is a frequently used measure of illness perception (IP). The aim of this study was to explore the psychometric properties of the questionnaire when used in elderly people with knee pain. Based on data from the Frederiksberg Cohort on elderly people reporting knee pain (N = 836), the psychometric properties of the eight B-IPQ items (1 'Consequences', 2 'Timeline', 3 'Personal control', 4 'Treatment control', 5 'Identity', 6 'Concern', 7 'Coherence', and 8 'Emotions') were analysed using Rasch analysis to establish whether the questionnaire provides reliable and valid measures of IP. Threshold disordering was found on the 1–10 rating scale in all items. When rescaling to a 0–2 rating scale, disordering was resolved in six items. Item goodness-of-fit analyses revealed that two items displayed underfit misfit and four items overfit misfit; hence, the B-IPQ does not present unidimensionality. The person separation index indicated that items separate respondents into only two IP levels. Finally, floor and ceiling effects were found, indicating that the B-IPQ may not fully describe the extent of IP in this population. The 1–10 rating scale used in the Danish B-IPQ targeting knee pain is suboptimal, while a 0–2 scale improves the psychometric properties of the scale. The B-IPQ does not present unidimensionality and the use of a sum score is therefore not recommended when assessing IP. The B-IPQ may not cover the extent of IP in elderly people with knee pain. [ABSTRACT FROM AUTHOR]

To evaluate the relationship between self-reported and performance-based measures of functioning in rheumatoid arthritis (RA), knee osteoarthritis (OA), and fibromyalgia (FM), and the influence of pain and fatigue. Self-reported functioning was assessed by the Stanford Health Assessment Questionnaire, Fibromyalgia Impact Questionnaire, and Knee injury and Osteoarthritis Outcome Score. Performance-based measures of task-related physical activity included grip strength and Six-Minute Walk Test (6MWT). Assessment of Motor and Process Skills (AMPS) was used to obtain performance-based measures of activities of daily living (ADL) ability. Pain and fatigue were assessed by 100 mm visual analogue scales. Spearman's rho correlation and regression modelling were applied. Correlations between self-reported functioning and performance-based measures of ADL ability were weak to moderate, and strongest in OA (r = 0.57, p = 0.002), and AMPS ADL ability measures did not enter regression models as explanatory factors for self-reported functioning. Correlations between AMPS ADL ability measures and measures of task-related physical activity were weak, except for a strong correlation between AMPS ADL motor ability and 6MWT in OA (r = 0.63, p = 0.000). The 6MWT was the only performance-based test explaining variance in AMPS motor ability (OA = 42%; FM = 11%). Pain explained variance in self-reported ability and contributed to variance in AMPS ADL motor ability measures in OA. Self-reported and observed measures of functioning assess partly different aspects of functioning, and both approaches may therefore be relevant in a structured assessment of patients with musculoskeletal disorders. [ABSTRACT FROM AUTHOR]