PHYS THER
Vol. 86, No. 11, November 2006, pp. 1498-1500
DOI: 10.2522/ptj.20060002.ar
Author Response
Paul W Stratford,
Deborah M Kennedy and
Linda J Woodhouse
We thank Dr Beaton for her stimulating commentary, which conveys important information concerning statistical modeling and conceptual frameworks relevant to the assessment of health outcomes. Her commentary includes general insights and issues specific to our article. Beaton has organized her review under 2 headings, and we will apply the same 2 headings in response to her comments.
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The Art of Statistical Modeling
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We agree with Beaton's position that art complements the science of statistical modeling in general and structural equation model specifically. Beaton wonders how information concerning a single-factor model or a model that includes the cross-loading of items on both factors would affect our conclusions. Our rationale for starting with a 2-factor model without cross-loading was that we conceptualized 2 factors with the relevant items loading uniquely on their respective factors. Although we did not desire cross-loading at the item-factor level, we imagined the factors would be related and allowed for a correlation between the factors. However, in response to Beaton's query regarding additional factor loading models, we will briefly summarize the results from 2 additional analyses. The first was a confirmatory factor analysis that specified a single factor with all pain and time items loading on this factor. The chi-square value indicated a poorly fitting mode (
2=419.4, df=20, P<.001) with the following factor loadings: self-paced walk–pain=.49, stair test–pain=.44, Timed "Up & Go" Test–pain=.49, Six-Minute Walk Test–pain=.34, self-paced walk–time=.94, stair test–time=.88, Timed "Up & Go" Test–time=.92, and 6-Minute Walk Test–distance=–.84. The negative loading with distance occurs because greater distances are associated with more desirable health status levels, whereas faster times and lower pain levels are related to more desirable health status levels. Our initial 2-factor model, which specified pain and time (distance) items loading on their respective factors, yielded a chi-square value of 70.7 on 19 degrees of freedom. The difference in chi-square values between the single-factor model and our initial 2-factor model is highly significant in favor of the 2-factor model (2= 348.7, df=1, P<.001).
The second supplementary analysis did not assume an a priori factor structure and allowed for all possible cross-loadings. We accomplished this by applying exploratory factor analysis using maximum likelihood estimation and oblique rotation. Two factors had eigenvalues greater than 1. Time/distance items loaded highly on one factor, and pain items loaded highly on the other factor. The correlation between factors was .46. The Table reports a summary of the pattern matrix factor loadings. It is evident from the Table that cross-loadings on the no-congruent factor were negligible. In summary, these supplementary analyses further support a 2-factor model that does not include a cross-loading of items.
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Conceptual Frameworks
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We have divided Beaton's comments into the following 3 topics and respond to each in turn: (1) our application of Bellamy's definition of lower-extremity physical function, (2) the proposition that the distinction between pain and function may diminish as the contextual complexity of an item increases (framed in the International Classification of Functioning, Disability and Health [ICF] lexicon, participation items are more contextually complex than activity items), and (3) the proposition that pain may be better assessed by applying nonattributed measures compared with attributed (eg, task-specific) measures of pain.
Bellamy is one of the few measure developers to offer a definition or clarifying phrase for the concept of lower-extremity functional status: "by this we mean your ability to move around and to look after yourself."1,2 We applied Bellamy's definition; however, we agree with Beaton's point that, in the context of our assessment process, the performance measures focus on "the ability to move around" and not on "the ability to look after yourself." Thus, within the ICF classification scheme, our performance measures assess aspects of activity limitation and not participation restriction.
The second point offered by Beaton is the speculation that the distinction between pain and function may become less distinct as the contextual complexity of an item or performance task increases. We are not aware of a study that has explored this exciting hypothesis prospectively. However, in a previous study that attempted to explain why the WOMAC physical function subscale could not detect deterioration identified by performance measures in patients 2 weeks after hip or knee arthroplasty, physical function items were divided into 2 sets.3 One set contained items similar to those on the WOMAC pain subscale, and the second set consisted of items not specific to the items on the WOMAC pain subscale. Item scores within each set were summed to yield total scores for the similar and not-specific item sets. The total item score for the not-specific item set detected deterioration consistent with the times from the performance tests. The item structure of the 2 sets of items is pertinent to our current discussion. All items in the set that did not detect deterioration tapped activity limitation; all but one item in the set that detected deterioration assessed participation restrictions. Although the results of this study are at odds with Beaton's hypothesis, the study was not conceived to investigate Beaton's hypothesis. We strongly support the need to investigate prospectively the hypothesis raised by Beaton. As a minor point, Beaton has suggested that our stair climbing test may be more contextually complex; however, given the way this activity was framed in our investigation—without any more relevance to self-care, occupation, and recreation than the other 3 performance tests—we are uncomfortable accepting this specific example.
Beaton's third point addresses the assessment of pain. Given the body of evidence suggesting that self-report measures of physical function are strongly influenced by pain,4–6 we examined whether a more distinct assessment of these 2 attributes could be achieved by performance measures. Our goal was to determine whether performance measures could bring into focus 2 health concepts that to date have been blurred by self-report measures. We believe that complementing, not replacing, self-report functional status and pain measures with performance measures will increase the validity of impressions of pain and physical function formed by clinicians. We concur with Beaton's point that pain is a complex health concept and that further investigation concerning attributed and nonattributed pain ratings is warranted in a larger sample of patients.
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References
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- Bellamy N.
An Evaluative Index for Clinical Trials. Hamilton, Ontario, Canada: McMaster University; 1982.
- Bellamy N.
WOMAC Osteoarthritis Index User Guide IV. Herston, Queensland, Australia: University of Queensland; 2000.
- Stratford PW, Kennedy DM. Does parallel item content on WOMAC’s Pain and Function Subscales limits its ability to detect change in functional status?
BMC Musculoskelet Disord. 2004;5:17.[CrossRef][Medline]
- Maly MR, Costigan PA, Olney SJ. Determinants of self-report outcome measures in people with knee osteoarthritis. Arch Phys Med Rehabil. 2006;87:96–104.[Web of Science][Medline]
- Terwee CB, van der Slikke RM, van Lummel RC, et al. Self-reported physical functioning was more influenced by pain than performance-based physical functioning in knee-osteoarthritis patients. J Clin Epidemiol. 2006;59:724–731.[CrossRef][Web of Science][Medline]
- Stratford PW, Kennedy DM. Performance measures were necessary to obtain a complete picture of osteoarthritic patients. J Clin Epidemiol. 2006;59:160–167.[CrossRef][Web of Science][Medline]
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Copyright © 2006 by the American Physical Therapy Association.
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Conceptual Frameworks
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