Demographic variables
The demographic information collected in this study included age (in years), sex, ethnicity (Singaporean Chinese, as compared with others), and education (1 = primary school or less, 2 = vocational or secondary school, 3 = college or university). Data about comorbidities were retrieved from patients’ medical records by a research assistant using a checklist modeled after the Self-Administered Comorbidity Questionnaire [17]. Patients’ height and weight were also obtained and body mass index (BMI) was calculated as the ratio of weight to squared height (kg/m2).
Self report physical function
At each assessment, patients were interviewed in either English or Mandarin using the Short Form-36 [18], of which we used the physical function subscale as the outcome variable and the bodily pain and mental health subscales as the study covariates. All subscales range from 0–100, with higher scores representing better health state. The English and Chinese versions of the SF-36 have been previously validated for use in Singapore [19].