Original ArticleThe SF-36 physical and mental health factors were confirmed in cancer and HIV/AIDS patients
Introduction
The RAND-36/SF-36 is a widely used generic health-related quality-of-life measure [1], [2], derived from the battery of items included in the RAND Medical Outcomes Study [3]. The 36 items were selected to ensure coverage of the full spectrum of physical and mental health (PH and MH) and they were grouped into eight scales: physical functioning (10 items), role limitations due to physical health problems (4 items), pain (2 items), general health perceptions (5 items), emotional well-being (labeled “mental health” in SF-36; 5 items), role limitations due to emotional problems (3 items), social functioning (2 items), and energy/fatigue (labeled “vitality” in SF-36; 4 items). One item about health change over the past year is not scored. The differences between the RAND-36 and SF-36 are the scoring algorithm applied when calculating two of the eight-scale scores and the composite (summary) scores.
The PH and MH summary scales were derived from the eight scales using factor analyses [1], [4], [5], [6], [7], [8]. The PH and MH factors identified by Ware et al. [8] were based on orthogonal rotation, whereas those identified by Hays et al. [1], [5] were based on oblique (nonorthogonal) rotation. In the SF-36 scoring system, all eight-scale scores are used in calculating the physical and mental component summary scores [2]. In contrast, the RAND-36 scoring system derives a PH composite score from four of the eight scales (physical functioning, role limitations due to PH problems, pain and general health perceptions) and a MH composite score from the other four scales. Recently, a scoring system that used all eight-scale scores to estimate correlated PH and MH factors was proposed [9].
Because of its extensive use, it is important to examine the dimensionality of the SF-36 in a variety of populations and using different approaches. This study evaluated the performance of the SF-36 items using an item response theory (“Rasch”) measurement model [10]. Specifically, we used Rasch residual factor analysis [11] to assess the extent to which items measure PH and MH as implied by the existing scoring systems.
Section snippets
Participants
The SF-36 data were collected as part of a larger project studying quality of life in cancer and HIV/AIDS patients [12]. Patients (N = 1,714) were recruited from five institutions. Of these, 56% were females; 81% Caucasians; and 15% African-Americans; mean age was 55.08 (SD = 14.76); and about 10% were from each of the major cancer sites (i.e., breast, colon, head and neck, lung and prostate) and HIV/AIDS.
Analysis
The 35-scaled items of the RAND-36 version 1 were scored so that higher values always indicate
Results
Abbreviated item content for the 35 scales items are listed in Table 1 along with the SF-36 scale they represent, summary measure allocation (PH vs. MH), item statistics, and factor loadings.
Physical and mental health
Two distinct but related dimensions (PH and MH) of self-reported health status have repeatedly emerged in studies by Hays et al. [16], in other analyses of the MOS [1], [6] and in other sample [17], [18], [19], [20]. Hays et al. [4] found that PH and MH were moderately correlated cross-sectionally and that both constructs were considerably stable over time. In some instances, dimensions of health have been identified that included one or more dimensions specific to a given patient population;
Acknowledgments
This study was supported by the National Cancer Institute (R01CA60068). Ron D. Hays, Ph.D., was supported in part by the UCLA/DREW Project EXPORT, National Institutes of Health, National Center on Minority Health & Health Disparities, (P20-MD00148-01) and the UCLA Center for Health Improvement in Minority Elders/Resource Centers for Minority Aging Research, National Institutes of Health, National Institute of Aging, (AG-02-004).
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