title: Measuring Physical Capacity.
abstract: OBJECTIVE
  The objective of this study was to develop and assess a composite measure of physical capacity using self-report and physical performance items.
  
  
  METHODS
  Item response theory (IRT) is used to evaluate measurement properties of self-report and performance items and to develop a composite measure for 7,609 participants in the National Health and Aging Trends Study.
  
  
  RESULTS
  Self-reports distinguish differences at the lower end of physical capacity but not at mid-to-high levels. Performance-based measures discriminate across a fuller spectrum. An IRT-based composite score, drawing on both, provides increased measurement precision across the physical capacity spectrum and detects age group differences if either self-report or performance does so-suggesting it is better suited for studying age-related changes than either measure alone.
  
  
  CONCLUSIONS
  Self-report and performance measures have different strengths on the physical capacity spectrum. IRT provides a means of combining these different measurement approaches for analyses of physical capacity across a broad range of functioning in later life.
authors.affiliation: 1 Johns Hopkins Bloomberg School of Public Health, Baltimore, MD, USA.
authors.forename: Judith D
authors.initials: JD
authors.lastname: Kasper
authors.affiliation: 1 Johns Hopkins Bloomberg School of Public Health, Baltimore, MD, USA.
authors.forename: Kitty S
authors.initials: KS
authors.lastname: Chan
authors.affiliation: 2 University of Michigan, Ann Arbor, USA.
authors.forename: Vicki A
authors.initials: VA
authors.lastname: Freedman
mesh_terms: D000368:Aged; D000369:Aged, 80 and over; D004185:Disability Evaluation; D005260:Female; D006304:Health Status; D006801:Humans; D008297:Male; D010809:Physical Fitness / Q000502:physiology*; D011594:Psychometrics; D057566:Self Report / Q000592:standards; D013647:Task Performance and Analysis
keywords: measurement; performance tests; physical capacity; physical function; self-reported function