A comparison of primary and proxy respondent reports of habitual physical activity, using kappa statistics and...

Many epidemiological studies rely in part on proxy informants. There is little published information on the reliability of proxy-respondent reports of physical activity.

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.

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.

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.