Measuring health-related quality of life: psychometric evaluation of the Tunisian version of the SF-12 health survey

Abstract

The 12-item short-form health survey (SF-12) was developed as a shorter alternative to the SF-36 for use in large-scale studies as an applicable instrument for measuring health-related quality of life. The main purpose of this study was to evaluate the psychometric properties of the Tunisian version of the SF-12. A stratified representative sample (N=3,582) of the general Tunisian population aged 18years and over was interviewed. SF-12 summary scores were derived using the standard US algorithm. Factor analysis was used to confirm the hypothesized component structure of the SF-12 items. Reliability was estimated using internal consistency, and construct validity was investigated with "known groups" validity testing and via convergent and divergent validity. SF-12 summary scores distinguished well, and in the expected manner, between groups of respondents on the basis of gender, age, education and socioeconomic status, thus providing evidence of construct validity. Mean scores in the total sample were 50.11 (SD 8.53) for the physical component summary (PCS) score and 47.96 (SD 9.82) for the mental component summary (MCS) score. The results showed satisfactory internal consistency and acceptable convergent validity for both summary scores. Cronbach's α coefficient for PCS-12 and MCS-12 was 0.73 and 0.72, respectively. Known groups comparison showed that the SF-12 discriminated well between groups of respondents on the basis of gender, age, education and socioeconomic status. In addition, no floor or ceiling effects at baseline were observed. The PCA confirmed the two-factor structure of the SF-12 items. Items belonging to the physical component correlated more strongly with the PCS-12 than those with the MCS-12. Similarly, items belonging to the mental component correlated more strongly with the MCS-12 than those with the PCS-12. The findings suggest that the SF-12 appears to be a valid and reliable measure that can be used for measuring of population health status. However, for optimal measurement, modifications to traditional scoring methods for the SF-12 should be considered.