The results of this study indicate that the measurement properties of the SF-12 items are remarkably similar for persons with and without a history of stroke in the context of a 2-factor measurement model that postulated separate but correlated physical and mental health factors. To our knowledge, this is the first study that has examined the factor structure of the SF-12 items among persons with a history of stroke using CFA. A prior study that investigated the “factor structure” of the SF-12 in stroke patients used a more exploratory principal components analysis approach [10
], and other researchers who have examined the SF-12's measurement properties among other patient populations have also utilized more exploratory factor analysis procedures [31
The excellent fit of the 2-factor model, the similarity of the estimated factor loadings across persons with and without a history of stroke, and the close correspondence between the sample-specific latent factors and the PCS and MCS scores as calculated with the standard scoring algorithms combine to strongly support the validity of the SF-12 and its component summary scores for assessing general HRQoL in research and clinical work with persons who report a history of stroke. Presumably this would also include those who are still recovering from recent stroke events. These findings are particularly noteworthy in light of the concerns raised by Hobart and colleagues [13
], who characterized the component summary scores provided by the SF-36 and, by extension, the SF-12, as having limited validity as measures of health outcomes following stroke. Our results demonstrate that SF-12 items have very similar psychometric scaling properties in the context of this 2-factor model for persons with and without a self-reported history of stroke, thus providing support for the validity of the SF-12 composite scores for measuring HRQoL in both groups. Other studies have also reported satisfactory performance of the SF-12 as an index of HRQoL among stroke patients [3
]. Relatively brief measures of HRQoL are needed in many situations, particularly in large-scale epidemiologic studies such as REGARDS where instrument length and ease of administration are key considerations [2
]. It is encouraging that, among persons with a history of stroke, a popular and standardized instrument such as the SF-12 appears to have psychometric properties that are comparable to those observed in the general population.
A moderate correlation was observed between the physical health and mental health latent factors, indicating that overall physical health is not completely independent of mental health. This moderate correlation is inconsistent with the near zero correlation between the SF-12 PCS and MCS scores, which is the result of a scoring algorithm based on an orthogonal factor rotation procedure [2
]. Results from prior studies have also questioned the forced orthogonality of the SF-12 and SF-36 summary scores [35
]. This artificial orthogonality of the PCS and MCS scores may undermine the sensitivity of these summary scores to capture veritable improvements in physical and mental health over time. Others have also shown that correlated PCS and MCS summary scores more accurately reflect the underlying raw data compared to orthogonal summary scores [26
]. Interestingly, Wilson and colleagues [39
] demonstrated that, of three factor extraction methods tested, SF-36 summary scores derived from a confirmatory factor analysis with correlated physical and mental health factors provided the best fit to the data.
Comparisons between persons with and without a history of stroke revealed that those with a history of stroke reported poorer physical and mental health compared to those without this self-reported history. These effects were found on both the REGARDS-specific latent factors and the standard SF-12 component summary scores. Both latent and component summary measures estimated the impact of stroke on physical functioning to be about twice as large as its impact on mental functioning. Prior investigators have similarly found evidence of diminished physical and mental health and HRQoL among persons who have experienced a stroke. For example, Carod-Artal and colleagues [41
] reported that stroke patients continued to report significant levels of depression, restriction in psychosocial functioning, and dependence in activities of daily living one year post stroke. Other studies have documented similar deficits as far out as four years post stroke [42
]. Previous analyses of REGARDS data have shown that even persons who report symptoms suggestive of stroke but deny a history of stroke or any transient ischemic attacks (included as persons without a history of stroke in the present analyses) report poorer HRQoL compared to symptom-free persons [6
]. Taken together, these findings suggest that even relatively mild strokes can have adverse effects on a person's physical and mental health, including emotional difficulties caused by the awareness of an increased risk for future strokes [43
Limitations of the present study include the reliance on self-report data for a positive history of stroke. Even so, prior epidemiological studies have demonstrated the validity of self-report in establishing a history of stroke [44
]. The SF-12 was only administered on a single occasion in this study, so we were unable to examine other potentially important SF-12 psychometric properties such as test-retest reliability or sensitivity to change over time. However, results from other studies suggest that the SF-12 summary scores have satisfactory test-retest reliabilities among stroke patients [4
]. We also acknowledge that, because respondents were non-institutionalized and appeared competent to provide informed consent, these findings may not be generalizable to all stroke survivors. Finally, we note that because the development model fit statistics have largely occurred within the context of parametric maximum likelihood estimations, their application to non-parametric ordinal models should be made cautiously. Aside from these limitations, we note that this paper is unique in several ways. It was conducted with a large national sample of persons who were not selected because of their history of stroke or stroke-related impairments and includes large numbers of participants with and without a self-reported history of stroke. This contrasts with prior studies that have largely relied on convenience samples from medical clinics or rehabilitation programs to examine questions concerning HRQoL following stroke.
In summary, this is the first study to our knowledge that has used CFA methods to examine the hypothesized 2-factor structure of the SF-12 items for persons who report a history of stroke. We demonstrated that this 2-factor model provides very good fit to the observed data and shows remarkable similarity across persons with and without a history of stroke. These findings support the validity of the SF-12 summary scores for assessing general HRQoL in persons with a history of stroke, and the use of this relatively brief instrument could yield useful data, especially in large sample observational studies where questionnaire administration time and reduction of research burden are important considerations. Finally, our findings argue that, to be valid, an evaluation of an instrument's psychometric properties must be driven by adequate statistical methodology, include an appropriate comparison group, and use sufficiently large samples. This is of considerable clinical, research, and public policy interest due to the potential for disservice when otherwise bona fide instruments are rejected on the basis of conclusions reached using less than adequate methodologies.