Background and objective: The SF-36 is one of the most widely evaluated and used quality of life questionnaires. Two summary components (physical and mental) have been developed from its 8 original dimensions. The objective of this study was to obtain the Spanish weights to calculate the summary components of the SF-36 and the SF-12, to evaluate their metric characteristics, and to obtain general population based reference norms.
Material and method: Representative samples of the Spanish population gathered from 2 cross-sectional studies: <<Drug consumption home survey>> (n = 9151) and <<Arterial hypertension and other risk factors in the elderly>> (n = 3,004). The Spanish weights of the summary components of SF-36 and SF-12 were calculated by principal component analysis and multiple linear regressions, respectively. Known groups were compared to evaluate construct validity and reliability was assessed with internal consistency coefficients.
Results: The Spanish weights obtained were very similar to those of the original American version (correlation > 0.9). The SF-12 items explained a 91% of the variance of SF-36 summary components. All the scores were lower (worse) in the elderly and most underprivileged groups (p < 0.001). Internal consistency coefficients were close to 0.9 for the SF-36, and slightly lower for the SF-12.
Conclusions: Physical and mental summary components estimated with Spanish weights are reliable and valid. The Spanish general population based norms of these summaries will facilitate the interpretation of quality of life results assessed with the SF family of questionnaires in Spanish studies.