In the economic evaluation of health interventions, the quality adjusted life year (QALY) can be used to measure outcomes. The QALY combines length and quality of life into a single figure. The quality aspect (or utility value) is anchored on a 0 (dead) to 1 (full health) scale, and can be derived from generic preference-based measures (GPBM) of health.
One such GPBM is the SF-6D (hereon SF-6Dv1)1,2; which was developed from version 1 of the SF-36.3 The SF-6Dv1 describes health on 6 dimensions [physical functioning (PF), role limitations (RL), social functioning (SF), pain, mental health (MH), and vitality (VT)], 4–6 severity levels, therefore describing 18,000 health states (Fig. 1). The United Kingdom value set was developed using the standard gamble elicitation technique and ranges from 0.29 to 1.1
;)
FIGURE 1: SF-6D health state classification system comparison.
The SF-6Dv1 has become one of the most widely used GPBMs in economic evaluation.4 Country specific value sets have been developed5–12 and it is accepted by international reimbursement agencies.13 SF-6Dv1 has been shown to have psychometric validity and responsiveness to change across common mental health14,15 and physical health conditions.16,17
The SF-6Dv1 has not been without criticism. The severity ordering of the PF dimension (between “a lot” of limitations with moderate activities and “a little” limitation with bathing and dressing) is unclear.1 The VT dimension is positively framed in comparison to the other dimensions. This may cause respondents confusion during valuation. The role dimension has limited sensitivity as it was based on combinations of 2 role items each with only 2 response levels. This resulted in claims of a “floor” effect, with many patients answering at the lowest severity level.18,19 The SF-6Dv1 was also developed using SF-36v1, and there is the opportunity to revisit the classification system using the improved SF-36v2.20,21
Moreover, there were additional concerns raised with regard to the valuation task used to derive the SF-6Dv1 value set. On one hand, the valuation technique used for SF-6Dv1, the standard gamble is a cognitively difficult technique, and given the iterative nature of the risk trade off concerns have been raised about respondent understanding of probability and risk aversion, which may lead to higher health state values. Furthermore, the valuation task involved a 2 stage chained process with states being valued against full health and the worst state, then the worst state being valued against full health and dead, which generates higher values by doubling the impact of risk aversion. Valuation using ranking,22 Bayesian methods23 and discrete choice experiments, including duration (DCETTO),13 have all produced lower values for the more severe states resulting in a wider utility range.
We therefore aimed to address these concerns by developing a new version of the SF-6Dv1 (SF-6Dv2). This includes 2 objectives: (1) to develop a “new” health state classification system from the SF-36v2 using psychometric evidence; and (2) to derive a value set that can be used in the calculation of QALYs. This paper reports on the first objective. The second objective is reported elsewhere.24
METHODS
Overview
We built on SF-6Dv1 and developed a new health state classification system that reflects the content of the SF-36, and produced health states amenable to valuation. To do this we used a 3-step process. Step 1 evaluated the dimensionality of the SF-36 for use in the classification system; Step 2 involved item elimination and selection; and Step 3 involved further analyses of the robustness of the Step 2 results across different data subsets. The dimension formation and item selection criteria used Factor and Rasch analyses, and other criteria such as cross-cultural relevance. Contrary to SF-6Dv1, item selection was not restricted to those also included in SF-12. This iterative methodological process was developed by a number of the authors, and has been applied widely to generate classification systems from existing profile measures.25–27
The SF-36
SF-363,28 is a measure of health that has been widely used and validated. It has 36 items across 8 dimensions: PF, role limitations due to physical health (RP), bodily pain (BP), general health (GH), VT, SF, role limitations due to emotional problems (RE), and MH. The SF-36v2 was used to develop SF-6Dv2. The SF-36v2 is an improved version of the SF-36v1. Changes included increasing the RP and RE item response levels from 2 to 5 to improve sensitivity, and simplifying the MH and VT items by reducing the levels from 6 to 5. Wording changes were also made.20,21
Data
Data used for this study were sourced from the 2 samples described below.
Health Outcome Data Repository Dataset29
Health Outcome Data Repository Dataset (HODaR) is a survey of recently discharged hospital inpatients and outpatients in the United Kingdom. The data included 49,029 full completers of the SF-36v2 between August 2002 and November 2008. The SF-36v2 was administered postally ∼6 weeks after discharge, with other information linked from hospital records.
Multi Instrument Comparison Study30
Multi Instrument Comparison (MIC) is a survey of respondents self-reporting a range of health conditions, and a “healthy public” sample. The data used were from 5,331 respondents who fully completed the SF-36v2 online in the United Kingdom (n=1358), Canada (n=1335), Australia (n=1171), and the United States (n=1467).
The 2 samples covered a wide range of conditions and included a large proportion reporting comorbid health problems. Table 1 reports demographics and Appendix 1 (Supplemental Digital Content 1, https://links.lww.com/MLR/C7) provides detailed descriptions of the datasets.
TABLE 1: Summary of HODaR and MIC Datasets
Step 1—Dimensionality Assessment
We determined the dimensions to include in the classification system by evaluating the dimensionality of the SF-36v2 using exploratory (EFA) and confirmatory (CFA) factor analysis. This was done considering the SF-6Dv1 6-dimension structure, and we looked for evidence supporting the inclusion of more or fewer dimensions. Past work assessing SF-36 dimensionality was also considered.31 This included research both supporting the hypothesized 8 factor structure,32–34 and suggesting that the direction of the item response wording may impact dimensionality assessment due to response set patterns.35,36 Work from Asia suggesting that the physical and emotional role dimensions load as one factor was also considered.37
The 5 GH items, and the health transition question, were not included as these are not relevant for a classification system assessing specific constructs of health. Factor analysis was conducted using Stata 15.38
The decision process used to select the dimensionality was also informed by face validity, conceptual coverage and cross-cultural issues. This was supported by input from the SF-6Dv2 international project team (including experts from 17 countries).
Exploratory Factor Analysis
EFA was used to identify patterns of loadings and examine dimensionality without assuming a prior structure by assessing the degree to which the item correlations can be explained by a number of factors. The number of factors to extract can be decided using eigenvalue analysis, or set by the analyst with consideration of the variance explained. We assessed the Kaiser-Meyer-Olkin measure of sample adequacy, which ranges between 0 and 1, where smaller values indicate that EFA may be inappropriate.
We extracted models with 2 to 9 factors (including the model suggested by the criteria of accepting factors with eigenvalues >1) on the full samples from both datasets. To ensure reliability, models were also tested on randomly selected subsamples. For each model we assessed the adequacy of the conceptual structures for developing a classification system.
Models were developed using oblique promax rotation, which assumes dimension correlations. We used polychoric correlations due to the ordinal responses. Items loading on a dimension with a correlation of <0.4, or cross-loading between dimensions within 0.2, were identified as demonstrating poor fit, but were not excluded from the model given the aim to develop a conceptually relevant classification system. This was in line with other studies developing classification systems.25–27
Confirmatory Factor Analysis
CFA39 assesses the fit of hypothesized factor structures by comparing the observed item correlation or covariance matrix with the expected matrix from the specified model. Analysis was conducted on the full sample from the HODaR and MIC datasets separately, with subsample analyses also performed. We used a range of statistics to assess model fit and guide dimensionality development. These included the root mean square error of approximation (RMSEA), and the standardized root mean square residual (SRMR) estimates of fit, where a value of. 08 or less indicated reasonable model fit and 0.05 or less indicated good fit.40,41 Two indices that take into account model fit and complexity, the comparative fit index (CFI)42 and the Tucker and Lewis43 Index (TLI), were also used. These range from 0 to 1, with values above 0.90 and 0.95 considered reasonable and excellent fit, respectively. These measures have been previously used to assess SF-12 and SF-36.44,45
The fit of 8 models was tested: a 7 factor model aligning with the SF-36 (PF, RP, RE, SF, BP, MH, VT for HODaR and MIC; models 1 and 2), the models produced using the selection criterion of eigenvalues >1 (models 3 and 4), and the best conceptually fitting models from EFA (models 5 and 6). CFA was also conducted on the model used for the classification system (models 7 and 8).
Step 2 and 3—Classification System Development
Rasch analysis46 models the relationship between categorical item responses in a multi item scale and a continuous latent scale which measures an assumed underlying unidimensional construct (in this case the aspect of health measured). As Rasch models assume unidimensionality, a separate model was evaluated for each dimension. The probability of a response to each level of each item was used to assess the severity of the item against the underlying latent construct. This allowed a range of item performance indicators to be assessed. Rasch analysis was conducted using RUMM2030.47
We evaluated Rasch fit statistics which assess the divergence between expected and observed responses for both respondents (person-fit residuals) and items (item-fit residuals). Items with a fit residual outside the standard cutoff of ±2.5 were considered for exclusion.
The Rasch model was also used to assess systematic differences in item response patterns for different subgroups of respondents [known as differential item functioning (DIF)]. DIF was detected using a 2-way analysis of variance assessment of the standardized residuals of the responses, where one factor was the class intervals representing severity across the latent trait scale and the other factor was the demographic subgroup.48 We tested for DIF based on age, sex and whether a health condition was reported. There are 2 types of DIF, uniform and nonuniform. Uniform DIF occurs when a subgroup consistently differs in their responses to an item conditional on the trait estimate. Presence of uniform DIF was indicated by a significant main effect for estimates for each item across demographic groups. Nonuniform DIF occurs when the association between item responses and group is not constant across the severity range. In the analysis this was indicated by a significant interaction effect between the subgroup and the severity range of the latent trait. A Bonferroni adjustment was applied to the significance estimates.
We additionally assessed items for inclusion in the classification system considering a range of criteria such as item severity range coverage, where a large distance from the lowest to the highest item response threshold indicates that an item provides measurement precision over a wider range.
Other criteria included avoiding complex item combinations and descriptions which may be difficult to understand and subsequently value. The face validity of the dimension wording was also evaluated.
Assessing the Robustness of the Results
To enhance robustness, Rasch and DIF analyses were conducted on 9 subsamples of combined HODaR and MIC data randomly selected for a sample of ∼500 (a recommended sample size for Rasch analysis).49 The subsamples did not significantly differ in terms of age, sex or health status. Each of the items was given an overall score (of 9) indicating the number of samples that the item performed well on. An item was considered for inclusion if it performed well on at least 5 of the 9 subsamples. However, in some cases items with lower performance remained for selection due to other criteria.
RESULTS
Step 1—Dimensionality Assessment
Exploratory Factor Analysis
Appendix 2 (Supplemental Digital Content 1, https://links.lww.com/MLR/C7) includes the EFA models with between 2 and 9 factors estimated on both datasets. The EFA models with between 2 and 6 dimensions combined a number of existing SF-36 dimensions within the factor structure, so were not suitable for developing a classification system. Models with 8 and 9 factors included redundant factors that were difficult to interpret and define.
The model including 7 factors was most in line with the original SF-36, and explained 75.1% (HODaR) and 67.8% (MIC) of the variance. The Kaiser-Meyer-Olkin of the models was high (>0.95) indicating that sample was adequate for factor analysis. Across the HODaR and MIC models, there were 5 factors consistent across the 2 datasets and 2 factors that included similar items with minor variation. Each factor is described below:
- Factor 1 (consistent): All 10 items from the PF dimension.
- Factor 2 (minor variation): Both models included the 4 RP items, and the HODaR model included the 2 SF items.
- Factor 3 (consistent): All 3 RE items.
- Factor 4 (consistent): 2 BP items.
- Factor 5 (minor variation): The HODaR model included the 5 MH items. The MIC model included the 3 negatively framed MH items.
- Factor 6 (consistent): The 4 positively framed items from the MH and VT. The 2 MH items cross-loaded with factor 5 in the HODaR data.
- Factor 7 (consistent): The 2 negatively framed VT items.
The SF dimension did not uniquely load on a single factor in either model, and failed to load at a level of 0.4 in the MIC model. The positively framed VT items loaded with the 2 positive MH items in MIC. The MH items loaded on one factor in the HODaR model, and on separate positive and negative factors in MIC.
Confirmatory Factor Analysis
Table 2 reports the CFA model fit. The worst performing models were those based on the factor structures produced using eigenvalue criteria (models 3 and 4). The models most consistent with the standard SF-36 structure (1 and 2) had CFI and TLI scores above the cutoff, but were below the models from the EFA [which would be expected given these are based on the same data samples (models 5 and 6)]. Models 1 and 2 performed better on the SRMR criteria in comparison to models 5 and 6, but lower RMSEA values were observed.
TABLE 2: Confirmatory Factor Analysis Model Fit Statistics
Decisions Regarding Dimensionality
PF explained the most variance and was retained. The role physical and role emotional dimensions were combined to create a single role dimension (RL) for 3 reasons. First, there is evidence from some Asian cultures that role limitations due to emotional problems is not recognized and so should not be separated out.37 Second, CFA modeling (Table 4, models 7 and 8) suggested that the correlation coefficients for the combined role dimension were acceptable. Third, including separate role physical and role emotional dimensions could complicate the valuation process. Although social limitations were correlated with RLs, the importance of general social activities with friends and family, and the likely impact of health interventions on social aspects, led to retaining this as an individual dimension assessing general SF. The 2 pain items formed a strong factor in both samples. These first 4 dimensions were negatively worded, and it was decided to ensure MH and VT were consistent with this. There was evidence from MIC that the positively worded items for MH overlap with VT. For MH the negatively worded items were part of the MH factor in HoDAR and formed a single factor in MIC. The negatively framed items were retained as a single MH dimension. The negatively framed VT items formed their own factor in both datasets, in line with earlier work,36 so were used as a simplified description of VT.
This 6 dimensional structure was in line with that used for SF-6Dv1. Table 2 reports the CFA model fit results based on the selected factor structure (models 7 and 8). These models had acceptable CFI and TLI scores. Model 7 had the lowest overall SRMR and model 8 had the lowest RMSEA. Table 3 reports the factor loadings from the CFA models for the final 6 factor model, and shows that all coefficients were above the minimum level. This provides support for retaining the 6 dimensional structure as the basis for the classification system.
TABLE 3: Confirmatory Factor Analysis Loadings for 6 Factor Model
Steps 2 and 3—Item Selection and Health State Classification System Development
The process for selecting items for each dimension is outlined below. Table 4 displays the results including the overall score for the number of times the item performed well of 9 subsamples, and scores regarding the times DIF or poor fit was exhibited. Appendix 3 (Supplemental Digital Content 1, https://links.lww.com/MLR/C7) includes an item-by-item summary of their performance, including the stage at which the item was excluded, and which were retained.
TABLE 4: Rasch Analysis, and Summary of Chosen Dimension Structure
Physical Functioning
Of the 10 items from the SF-36 PF scale, there was evidence of misfit for 4 items. Two items displayed DIF by sex.
Of the remaining 4 items, the vigorous activity and bathing and dressing limitation items were selected to ensure the dimension was sensitive to the less severe (mean latent scale coverage 1.63 to 3.78) and more severe (coverage −3.71 to −1.42) range, respectively. As these items did not overlap in values on the latent scale, the item assessing moderate activity limitations was chosen to cover the mid-range of the severity scale (mean range −0.88 to 1.64). This item did demonstrate misfit in a number of the analyses. However, retaining this item in the classification system increased sensitivity across the severity range.
Role Limitations
Across the role physical and role emotional dimensions, all 7 items had good Rasch statistics on the majority of analyses and covered a similar severity range. A key criterion was face validity, and the physical and emotional role dimensions were combined to tackle cross-cultural factors linked to evidence that RL due to emotional problems is not recognized in some Asian countries. Therefore, it was decided to use items assessing the same concept across both physical and emotional RLs. This resulted in the item “Accomplished less than you would like” being used for both but with attribution to either physical health or emotional problems.
Social Functioning
Both items performed well, and covered a similar severity range, with the average position centrally located on the logit scale (0.01 and −0.01). Because of this, and for consistency with SF-6Dv1, it was decided to retain the social activity limitation frequency item.
Pain
There was more evidence of misfit for pain interference (7/9 samples) compared to pain severity (4/9 samples). The pain severity item covered a wider range, particularly at milder severity. Furthermore, it avoided reference to interference with work and correlated less with the PF, RP, and RE dimensions. For these reasons pain severity was selected.
Mental Health
The item “down in the dumps” displayed item misfit. The negatively worded items for assessing frequency of being “very nervous” and “down and depressed” remained for selection following the Rasch analysis and they were combined as one dimension within the classification system.
Vitality
Both the negatively framed VT items performed well. The item assessing being “worn out” was selected over the more general and less severe “tired” item.
SF-6Dv2 Classification System
The SF-6Dv2 classification system is displayed in Figure 1, with SF-6Dv1 for comparison. The figure shows the differences in the items selected, and the simplification of the dimension level wording to support valuation. This was done through an iterative process, led by authors J.E.B., B.J.M., and D.R., of adapting the item content and item level wording into single sentences reflecting the original meaning of the item. The process involved review and revision of potential changes by the other authors and the international SF-6Dv2 team. Appendix 4 (Supplemental Digital Content 1, https://links.lww.com/MLR/C7) demonstrates how the selected SF-36v2 items were converted into the SF-6Dv2 classification system. In constructing the classification, the team was aware that the initial valuation will use DCETTO and this requires respondents to compare states varying in content. The simplification in wording was achieved by providing instructions at the beginning of the task that the descriptions are because of health.
The key changes in comparison to SF-6Dv1, are:
- PF uses the same items but reduces the levels from 6 to 5 to avoid ambiguity in level ordering by removing “Limited a little in bathing and dressing”.
- RL descriptions are simplified by using the same item to represent the 2 constituent dimensions. The increase to 5 levels takes advantage of the greater sensitivity of SF-36v2.
- SF is the same as for SF-6Dv1, with simplified level descriptions.
- Pain has changed from interference to severity to increase sensitivity to changes in pain intensity.
- MH includes both depression and anxiety focused items in line with SF-6Dv1.
- VT has changed from positively worded “energy” to negatively worded “worn out.”
CONCLUSIONS
In this study we have developed a new version of the SF-6D classification system (SF-6Dv2) informed by psychometric results and considering previous work testing the limitations of SF-6Dv1. This process means that the SF-6Dv2 should overcome many of the published criticisms of SF-6Dv1 but maintain similarities. This is because we have used the same dimension structure and retained much of the descriptive content, whilst simplifying wording to support valuation using DCETTO. We also involved international experts to ensure cultural issues were considered. Some exploratory comparisons of the SF-6Dv1 and SF-6Dv2 using the preferred value sets are reported elsewhere.24
There are improvements between versions to standardize the direction of the wording (VT), simplify the wording (RL), remove inconsistencies (PF) and move towards the measurement of pain severity (PA). Standardizing the direction of the wording has been shown to increase the psychometric validity of tests.50 Item level changes will impact on the ability to measure change in health status over time.
This study has a number of limitations and areas for future work. First, we used datasets that were restricted to westernized majority English speaking countries. To overcome this the international expert group included researchers involved in the original development and translation of SF-36. This was influential in the decision to combine the role dimensions. DIF analysis based on a wider range of demographics (such as language, education and country of residence) was not possible as we were restricted to the data collected in the HODaR and MIC studies. In contrast to the development of SF-6Dv1, we did not restrict the item selection to those included in SF-12. SF-6Dv2 values will be estimated from SF-12 using mapping algorithms in line with methods used elsewhere.51
Further work is required to psychometrically test SF-6Dv2 in comparison to SF-6Dv1, particularly the responsiveness of items to change. This will allow for further understanding of the new classification system to support widespread use of the instrument. A key comparison is with the EQ-5D-5L.52,53 There are important differences between the SF-6Dv2 and EQ-5D-5L descriptive systems that will result in different psychometric indicators. Comparisons in patient datasets will help understand the relationship between the measures, and inform the use of each in health technology assessment.
In conclusion, we have developed a simplified version of the SF-6D classification system considering many of the criticisms of SF-6Dv1. We have used updated psychometric methods that allow insight into the choices made during the development of SF-6Dv1. This led to the same dimension structure, but with improvements. The classification system will be valued internationally using valuation methods based on DCE. The United Kingdom valuation is reported in a companion paper,24 and subsequent international value sets can be tested for use in the estimation of QALYs for the economic evaluation of health interventions.
ACKNOWLEDGMENTS
The authors would like to acknowledge the passing of their dear colleague Barb Gandek during the final writing up of this research. She will be greatly missed by all. The authors would like to acknowledge the input of the remaining SF-6Dv2 international project team which includes: Nick Bansback, Beate Bestmann, Luciane Cruz, Rajabali Daroudi, Lara Ferreira, Pedro Ferreira, Shunichi Fukuhura, Lewis Kazis, Thomas Kohlmann, Maria Knoph Kvamme, Cindy Lam, Clara Mukuria, Richard Norman, Jan Abel Olsen, Julie Ratcliffe, Antonio Rosello, Akbari Sari, Rick Sawatsky, Elly Stolk, Dong Suh, Gemma Vilagut, David Whitehurst, Carlos Wong, Jing Wu, Yosuke Yamamoto. They would also like to thank the HODaR and MIC project teams for use of their data in this study.
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