Objective: To compare the discriminative power of the SF-6D index scores derived from the SF-36 (SF-6D36) and SF-12 (SF-6D12) in the general population.
Methods: Data from the National Health Measurement Study were used. The F statistic was used to compare the relative efficiency of the SF-6D36 and SF-6D12, as well as the EQ-5D, HUI2, and HUI3 index scores, in discriminating between respondents with and without 1 of the 11 chronic medical conditions. The efficiency of the multiattribute health classification systems of the study instruments was measured using the Shannon index (H'). The relative efficiency of the SF-6D36 and SF-6D12 was also compared in respondents who were on the ceilings of the EQ-5D, HUI2, and HUI3 scales.
Results: The SF-6D36 score was systematically lower than the SF-6D12 score at the group level (range, 0.022-0.036). The SF-6D36 exhibited higher discriminative power in 8 and 5 conditions than the SF-6D12 and all other index scores, respectively. The SF-6D36 had higher H' values than the SF-6D12 in the dimensions of physical functioning (1.73 vs. 0.78), mental health (1.70 vs. 1.39), and bodily pain (2.16 vs. 1.56) as well as than all other instruments in similar health dimensions. In respondents reporting full health on the EQ-5D, HUI2, or HUI3, the SF-6D36 better discriminated between those with and without medical conditions than the SF-6D12.
Conclusions: The SF-6D derived from the SF-36 is more discriminative than that derived from the SF-12 and is therefore preferred for use in population health surveys where a preference-based health index is needed.