Original article
Development of a Self-Report Physical Function Instrument for Disability Assessment: Item Pool Construction and Factor Analysis

Presented to the American Public Health Association, November 1, 2011, Washington, DC; the Psychiatric Research Center, Geisel School of Medicine, April 20, 2012, Lebanon, NH; and the International Association of Bodily Impairment, September 12, 2012, Montreal, QC, Canada.
https://doi.org/10.1016/j.apmr.2013.03.011Get rights and content

Abstract

Objectives

To build a comprehensive item pool representing work-relevant physical functioning and to test the factor structure of the item pool. These developmental steps represent initial outcomes of a broader project to develop instruments for the assessment of function within the context of Social Security Administration (SSA) disability programs.

Design

Comprehensive literature review; gap analysis; item generation with expert panel input; stakeholder interviews; cognitive interviews; cross-sectional survey administration; and exploratory and confirmatory factor analyses to assess item pool structure.

Setting

In-person and semistructured interviews and Internet and telephone surveys.

Participants

Sample of SSA claimants (n=1017) and a normative sample of adults from the U.S. general population (n=999).

Interventions

Not applicable.

Main Outcome Measure

Model fit statistics.

Results

The final item pool consisted of 139 items. Within the claimant sample, 58.7% were white; 31.8% were black; 46.6% were women; and the mean age was 49.7 years. Initial factor analyses revealed a 4-factor solution, which included more items and allowed separate characterization of: (1) changing and maintaining body position, (2) whole body mobility, (3) upper body function, and (4) upper extremity fine motor. The final 4-factor model included 91 items. Confirmatory factor analyses for the 4-factor models for the claimant and the normative samples demonstrated very good fit. Fit statistics for claimant and normative samples, respectively, were: Comparative Fit Index=.93 and .98; Tucker-Lewis Index=.92 and .98; and root mean square error approximation=.05 and .04.

Conclusions

The factor structure of the physical function item pool closely resembled the hypothesized content model. The 4 scales relevant to work activities offer promise for providing reliable information about claimant physical functioning relevant to work disability.

Section snippets

Methods

All study procedures were approved by the Boston University Internal Review Board.

Results

The flow of item pool development is presented in figure 1, beginning with 381 initial items identified from the literature review, of which 119 originated from PROMIS or Neuro-QOL systems. After cognitive interviewing, 10 items were rewritten and 8 were removed. The final item pool used for the calibration field test is shown in figure 2, consisting of 139 items, including 24 items specific to wheelchair and walking aid use.

The characteristics of the claimant and normative samples

Discussion

This article reports on the initial stages of the development of an instrument to measure physical functioning, the Social Security Administration Physical Function Instrument.

Factor analyses of the item pool using data from a sample of 1017 SSA claimants and a normative sample of 999 U.S. adults revealed a 4-factor structure. The practical implication of this finding is the construction of 1 scale for each of the 4 factors plus a wheelchair mobility scale. This will allow the user to obtain a

Conclusions

The current work integrates major advances in the conceptualization of disability and measurement methodology to provide a contemporary and comprehensive representation of physical functioning for use within the SSA disability programs. The development of the Social Security Administration Physical Function Instrument is a critical step toward the important goal of integrating standardized functional information into the disability determination process within SSA disability programs. The item

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    Supported by the Social Security Administration-National Institutes of Health Interagency Agreements under the National Institutes of Health (contract nos. HHSN269200900004C, HHSN269201000011C, HHSN269201100009I); the National Institutes of Health Intramural Research Program; and a New Investigator Fellowship Training Award from the Foundation for Physical Therapy.

    No commercial party having a direct financial interest in the results of the research supporting this article has or will confer a benefit on the authors or on any organization with which the authors are associated.

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