Abstract
BackgroundTo develop and validate an item bank to measure mobility in older people in primary care and to analyse differential item functioning (DIF) and differential bundle functioning (DBF) by sex.MethodsA pool of 48 mobility items was administered by interview to 593 older people attending primary health care practices. The pool contained four domains based on the International Classification of Functioning: changing and maintaining body position, carrying, lifting and pushing, walking and going up and down stairs.ResultsThe Late Life Mobility item bank consisted of 35 items, and measured with a reliability of 0.90 or more across the full spectrum of mobility, except at the higher end of better functioning. No evidence was found of non-uniform DIF but uniform DIF was observed, mainly for items in the changing and maintaining body position and carrying, lifting and pushing domains. The walking domain did not display DBF, but the other three domains did, principally the carrying, lifting and pushing items.ConclusionsDuring the design and validation of an item bank to measure mobility in older people, we found that strength (carrying, lifting and pushing) items formed a secondary dimension that produced DBF. More research is needed to determine how best to include strength items in a mobility measure, or whether it would be more appropriate to design separate measures for each construct.
Highlights
To develop and validate an item bank to measure mobility in older people in primary care and to analyse differential item functioning (DIF) and differential bundle functioning (DBF) by sex
Following the Rasch rating scale model (RSM) analysis, 35 items remained in the pool and formed the Late Life Mobility item bank (LLM-IB), which measured with a reliability of 0.90 or higher across the entire spectrum of mobility, except at the extreme end of better function
We have designed an item bank in Spanish to measure mobility in older primary care patients which is free from item bias across gender and was calibrated using Rasch RSM
Summary
To develop and validate an item bank to measure mobility in older people in primary care and to analyse differential item functioning (DIF) and differential bundle functioning (DBF) by sex. In addition to measuring physical function with fixed length scales such as the Health Assessment Questionnaire [2] or the subscale of physical functioning of the Medical Outcomes Study Short Form-36 (PF-10) [3], it can be measured using item banks based on item response theory (IRT) models [4,5]. In some of these item banks, physical function is measured as a two-dimensional construct consisting of mobility and upper extremity function [6,7], in others a unidimensional solution has been considered more appropriate [8,9]. Neither upper extremity function items nor disability (in activities of daily living) items were included
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