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.

Method: 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.

Discussion: 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.

Keywords: measurement; performance tests; physical capacity; physical function; self-reported function.

Figure 1

Comparison of walking performance test and self-report item (IRT item characteristic curves): (a) Performance Test Walking Speed Score (0,1,2,3,4); (b) Self-report Walking Item (0,1,2). Note. The x-axis represents levels of physical capacity where 0 is the group mean. Negative values represent levels of physical capacity below the group mean in SDs and positive values represent levels of physical capacity above the group mean in SDs. The y-axis represents the probability of endorsing a response: for performance items, scores of 0, 1, 2, 3, 4 shown as 5 lines starting from the left; for self-report items, scores of 0, 1, 2 shown as 3 lines starting from the left. Examples of interpretation: for persons in the physical capacity range at −2.0 (lower functioning at 2 SD below the mean), the probability of a score of 0 on the walking speed test is about 70% and the probability of a score of 1 is about 20%; on the walking self-report item, the probability of a score of 0 is virtually 100%. For persons in the physical capacity range at 2.0 (higher functioning at 2 SD above the mean), the probability of a score of 4 is a little over 80%, a score of 3 is about 15%, and there is a less than 5% chance of a score of 2; on the walking self-report item virtually all persons have a score of 2. IRT = item response theory.

Figure 2

Standard error of the score estimate for physical capacity using all items, performance-based items only, and self-report items only. Note. The x-axis represents levels of physical capacity where 0 is the group mean. Negative values represent levels of physical capacity below the group mean in SDs and positive values represent levels of physical capacity above the group mean in SDs. Standard Errors are shown on the y-axis where larger values represent larger standard errors. Standard errors are lower across the physical capacity spectrum for the measure using all items, particularly at the higher end relative to self-report only and from 1 SD below and above the mean relative to performance only.

Figure 3

Mean physical capacity based on performance tests, self-report items and IRT score by age group. Source. National Health and Aging Trends Study (NHATS; 2011). Note. N = 7,609 persons 65 years and older. Excludes nursing home residents. IRT = item response theory.