title: A comparison of primary and proxy respondent reports of habitual physical activity, using kappa statistics and log-linear models.
abstract: BACKGROUND
  Many epidemiological studies rely in part on proxy informants. There is little published information on the reliability of proxy-respondent reports of physical activity.
  
  
  METHODS
  Self-reported data on vigorous and moderate physical activity, from a representative sub-sample of participants in a community-based case-control study of coronary heart disease, were compared with information collected from their next-of-kin.
  
  
  RESULTS
  Relative to primary respondents, proxy respondents under-reported activity by approximately 10 percentage points, for both leisure and work-time activity. On a simple three point scale (inactivity/moderate activity/physical activity), 70% of primary-proxy pairs were in exact agreement with regard to leisure time activity and 67% of pairs were in exact agreement on work-time activity. The corresponding values for the weighted kappa statistic were 0.66 [95% confidence interval (CI) 0.59-0.72] and 0.62 (0.54-0.72). Log-linear modelling provided evidence for superior agreement on worktime activity when the proxy was not the primary respondent's spouse.
  
  
  CONCLUSIONS
  Overall levels of primary-proxy respondent agreement on physical activity seem somewhat lower than has been reported for smoking and alcohol-drinking frequency. There seems little reason to prefer spouse proxies when endeavouring to elicit information on work-time physical activity. Log-linear modelling provides an efficient means of exploring covariate effects in observer-agreement studies.
authors.affiliation: Department of Public Health and General Practice, Christchurch School of Medicine, New Zealand.
authors.forename: P
authors.initials: P
authors.lastname: Graham
authors.forename: R
authors.initials: R
authors.lastname: Jackson
mesh_terms: D000328:Adult; D016022:Case-Control Studies; D004812:Epidemiologic Methods; D005260:Female; D006801:Humans; D007899:Leisure Activities; D016014:Linear Models; D016015:Logistic Models; D008297:Male; D008875:Middle Aged; D015233:Models, Statistical; D009068:Movement; D009520:New Zealand / Q000453:epidemiology; D019727:Proxy; D011795:Surveys and Questionnaires; D014937:Work