Translation and performance of the Norwegian SF-36 Health Survey in patients with rheumatoid arthritis. I. Data quality, scaling assumptions, reliability, and construct validity.

The SF-36 was translated into Norwegian following the procedures developed by the International Quality of Life Assessment (IQOLA) Project. To test for the appropriateness of the Norwegian Version 1.1 of the SF-36 in patients with rheumatoid arthritis (RA), 1552 RA patients were mailed the form. Psychometric methods used in previous U.S. and Swedish studies were replicated. The response rate was 66%. The sample (mean age 62 years, mean disease duration 13 years) was over-represented by females (79%). Totally, 74% of the questionnaires were complete. Missing value rates per item ranged from 0.4% to 9.0% (mean 4.2%). In the Role-Emotional scale, all three items had missing value rates above average and higher than reported in the U.S. and Swedish studies. Tests of scaling assumptions confirmed the hypothesized structure of the questionnaire, but results were suboptimal in the General Health scale. In all scales the Cronbach's alphas exceeded the 0.70 standard for group comparisons. In the Physical Functioning scale, Cronbach's alpha exceeded the 0.90 standard for individual comparisons. There was good evidence for the construct validity of the questionnaire. Generally, the Norwegian SF-36 version 1.1 distributed to RA patients held the psychometric properties found in other countries and in normal populations. The translations of items in the General Health and Role-Emotional scales were reassessed. Minor deficiencies were detected and changed (SF-36 Norwegian Version 1.2).