AIMS: In item response theory, unidimensionality is an important assumption. Currently, most HRQOL questionnaires are multidimensional. The aim of this study was to assess the unidimensionality among scales from WHOQOL-BREF and SF-36 in chronic lung disease patients. METHODS: A 3-stage approach was used. In the first stage, a conceptual literature review was performed to compare the theoretical structures of these two questionnaires. In the second stage, exploratory factor analysis and correlation analysis were performed to identify groups of factors. In the third stage, the scaling and contents of each factor were discussed by experts to decide the groups of factors. A sample of 395 lung disease patients (235 males and 160 females, aged 15-66 years with a mean age of 42.8 years), identified in the 2001 National Health Bureau survey was used in our analysis. RESULTS: Our results indicated the unidimensionality of the WHOQOL-BREF physical domain, SF-36 BP and SF-36 VT (all paired correlation > .50), which was named 'physical' domain. There was also unidimensionality of the WHOQOL-BREF psychological domain and SF-36 MH (all paired correlation > .50), which was named the 'mental' domain. Cronbach's alpha coefficients for the two newly formed dimensions (physical and mental) are .90 and .85. Though SF-36 VT had a strong correlation with WHOQOL physical domain, VT was not included in the 'physical' domain because a dichotomous scale was used for this domain whereas the other domains in 'physical' domain used five or six scales. No correspondence could be found between the remaining five SF36 domains and the two remaining WHOQOL domains. CONCLUSIONS: The results may help researchers better understand the similarities and dissimilarities of WHOQOL-BREF and SF-36 and provide a basis for further IRT study.
