Using Confirmatory Factor Analysis (CFA) to Assess Structure of SF-36 (Persian Version) questions for normal people and patients with chronic diseases

Introduction: This study aims at analyzing factor structure of Short Form Healthy Survey (SF-36), Persian Version, for both normal people and patients with chronic diseases. Methods: In this cross-sectional study, a total of 585 patients with chronic diseases and 642 over 18-years old normal people were selected using simple casual sampling method. When the survey was completed by the participants, the Cronbach's alpha coefficient and confirmatory factor analysis (CFA) were used to determine the survey's validity and reliability. T-test also was used to compare the average score of the survey's domains in both groups. Results: For the group of patients with the chronic diseases, the highest mean belonged to the social functioning domain's score (43.11±18.48). In contrast, for the normal people's group, vitality found the highest mean score (50.83±9.95). T-test results showed that the mean difference in all domains except the role limitations because of social impediments for both normal people and patients was significant. Cronbach's alpha coefficient for all eight domains of the survey for the normal people varied from 0.68 to 0.90, which it indicated an acceptable reliability for the survey whereas the coefficient for the eight domains of the survey for patients varied from 0.66 to 0.91. CFA results using goodness-of-fit indicators showed that the 8-factor structure, suggested by the developer, is not verified for the patients group and the CFA results in the normal people's group showed that the 8-factor model has an acceptable fitness for the data. Conclusion: CFA is an effective method to analyze structure of different domains of SF-36 in various groups. For this reason, researchers need to be wary when they use the survey and also they need to check initially its reliability and validity in different sub-groups.