Predicting an SF-6D preference-based score using MCS and PCS scores from the SF-12 or SF-36

Abstract

Background: The SF-6D preference-based scoring system was developed several years after the SF-12 and SF-36 instruments. A method to predict SF-6D scores from information in previous reports would facilitate backwards comparisons and the use of these reports in cost-effectiveness analyses.

Methods: This report uses data from the 2001-2003 Medical Expenditures Panel Survey (MEPS), the Beaver Dam Health Outcomes Survey, and the National Health Measurement Study. SF-6D scores were modeled using age, sex, mental component summary (MCS) score, and physical component summary (PCS) score from the 2002 MEPS. The resulting SF-6D prediction equation was tested with the other datasets for groups of different sizes and groups stratified by age, MCS score, PCS score, sum of MCS and PCS scores, and SF-6D score.

Results: The equation can be used to predict an average SF-6D score using average age, proportion female, average MCS score, and average PCS score. Mean differences between actual and predicted average SF-6D scores in out-of-sample tests was -0.001 (SF-12 version 1), -0.013 (SF-12 version 2), -0.007 (SF-36 version 1), and -0.010 (SF-36 version 2). Ninety-five percent credible intervals around these point estimates range from +/-0.045 for groups with 10 subjects to +/-0.008 for groups with more than 300 subjects. These results were consistent for a wide range of ages, MCS scores, PCS scores, sum of MCS and PCS scores, and SF-6D scores. SF-6D scores from the SF-36 and SF-12 from the same data set were found to be substantially different.

Conclusions: Simple equation predicts an average SF-6D preference-based score from widely published information.

Figure 1

Distribution of SF-6D scores from all data sources. Data from 2002 MEPS were used for equation estimation and all other data were used for equation testing. (a) BDHOS SF-12v1. (b) NHMS SF-12v2. (c) BDHOS SF-36v1. (d) NHMS SF-36v2. (e) 2001 MEPS SF-12v1. (f) 2002 MEPS SF-12v1. (g) 2003 MEPS SF-12v2. BDHOS, Beaver Dam Health Outcomes Survey; MEPS, Medical Expenditure Panel Survey; NHMS, National Health Measurement Survey.

Figure 2

Average difference in observed and predicted SF-6D score and root mean squared error by age, mental component summary (MCS) score, physical component summary (PCS) score, and summed MCS and PCS score. Each point represents 500 groups of 50 observations that were randomly selected from the 2001 MEPS. Inclusion in the groups was constrained by either age, MCS score, or PCS score. The difference between the observed average SF-6D score of the group and the predicted average SF-6D score was calculated. This figure illustrates the mean of these differences with 95% confidence intervals by age strata (a), MCS score strata or PCS score strata (c), and sum of MCS and PCS score strata (e). This figure also illustrates the mean squared error of by age strata (b), MCS score strata or PCS score strata (d), and sum of MCS and PCS score strata (f).

Figure 3

Observed and predicted mean SF-6D scores for groups with 50 observations. This figure illustrates the mean observed and mean predicted SF-6D scores for groups of 50 observations were randomly selected from the 2001 Medical Expenditure Panel Survey. SF-6D group strata included 0.30–0.45, 0.325–0.475, 0.35–0.5, 0.375–0.525, 0.40–0.45, 0.425–0.575, 0.45–0.6, 0.475–0.625, 0.50–0.45, 0.525–0.675, 0.55–0.7, -0.575–0.725, 0.60–0.45, 0.625–0.775, 0.65–0.8, 0.675–0.825, 0.70–0.45, 0.725–0.875, 0.75–0.9, 0.775–0.925, 0.80–0.45, 0.825–0.975, 0.85–1.0, 0.875–1.0, 0.90–1.0, 0.925–1.0, and 0.95–1.0. Ten groups were randomly selected from each of the strata.