Original ArticleThe Factor Structure of the SF-36 Health Survey in 10 Countries: Results from the IQOLA Project
Introduction
According to cross-cultural psychometric research traditions, the degree to which universal concepts are captured by a measure can be evaluated by examining the extent to which measurement and structural models are replicated cross culturally 1, 2, 3, 4, 5, 6, 7. Measurement models concentrate on the scoring of response choices and hypothesized item groupings and are the basis for scale scoring algorithms. Structural models focus on the relationship of scales to each other and are useful in scale interpretation. They are the psychometric basis for scoring summary measures. To the extent that scales have the same relationships with other scales and with components of health across countries, the meaning and interpretation of scale scores and summary components are more likely to be comparable.
In its protocol for translating and validating the SF-36 Health Survey, the International Quality of Life Assessment (IQOLA) Project adopted a comprehensive three-stage research methodology that examines the extent to which the SF-36 measurement and structural models are replicated cross culturally [8]. Stages 1 (reproduction of the questionnaire) and 2 (reproduction of the scoring algorithms) address the issues of conceptual equivalence of the translation and satisfactory replication of the SF-36 measurement model. Stage 3 focuses on the structural model, including examination of the pattern of correlations among SF-36 scales and the relationship of scales to external variables. A number of studies have tested the structure of the SF-36 in other countries 9, 10, 11, 12, 13, 14, and other studies have evaluated its cross-cultural validity (see Gandek et al. [15] and other articles in this issue). This article uses traditional factor analysis methods to examine the equivalence of the SF-36 structural model in 10 countries. A companion article included in this issue uses a structural equation modeling approach [16].
The SF-36 measures a full range of health states and includes multi-item scales measuring each of eight health concepts: physical functioning (PF), role limitations due to physical health (RP), bodily pain (BP), general health (GH), vitality (VT), social functioning (SF), role limitations due to emotional problems (RE), and mental health (MH) 17, 18. These eight scales have been observed to define distinct physical and mental health clusters in factor analytic studies of both general and patient populations in the United States 19, 20, 21. The physical and mental health components accounted for more than 80% of the reliable variance in SF-36 scale scores and have been very useful in establishing interpretation guidelines for each of the SF-36 scales 19, 20. Results to date from published studies in Australia [9], Italy [10], the Netherlands [11], Sweden [12], Switzerland [13], and the United Kingdom [14] also provided considerable support for the construct validity of the SF-36.
In this article, principal components analysis was used to test the generalizeability of the two-dimensional SF-36 model using identical methods across nine Western European countries. Principal components analysis gauges the congruence between the hypothesized physical and mental health constructs and the SF-36 scales used to measure those constructs. To the degree that the two-dimensional structure is replicated across countries, the construct validity of the SF-36 and the structural model used in scoring summary health measures are valid across countries. The results also have implications for the interpretation of each SF-36 scale as a measure of physical or mental health.
Section snippets
Sample and Data Collection
SF-36 data came from general population surveys in Denmark, France, Germany, Italy, the Netherlands, Norway, Sweden, Spain, the United Kingdom, and the United States. Translations of the SF-36 were developed and tested using a standard methodology operationalized by the IQOLA Project [22]. The IQOLA methodology also specified a standard protocol for collection of normative general population data. Briefly, the protocol included guidelines for sample size, use of representative sampling of
Results
As hypothesized, eigenvalues for the first two components were generally greater than unity (Table 1). In Italy, Spain, and the United Kingdom, eigenvalues for the second component were slightly lower than unity (0.88 to 0.97). Across the 10 countries, 65.6% to 71.6% (median = 69.6%) of the total variance and 76.3% to 84.7% (median = 82.4%) of the reliable variance in SF-36 scale scores were accounted for by the two components.
The total and reliable variance explained in each SF-36 scale by the
Discussion
Overall, these results are consistent with those from other studies 9, 10, 11, 12, 13, 14 and strongly support the generalizeability of the two-dimensional (physical and mental health) model of the SF-36 across the nine Western European countries studied. As previously reported for the United States 18, 19, 20, the interpretation of the two derived components as dimensions of physical and mental health was straightforward and robust across countries and across age and gender subgroups within
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