Predicting risk among non-respondents in prospective studies.

Potential non-response bias was investigated in a follow-up study of 2,011 chronically disabled patients. 82.5% and 73.3% of the study subjects responded to self-administered mail questionnaires respectively at 6-month and 1-year follow-up. Information on employment status, the outcome of interest, of approximately 90% of the non-respondents was obtained from indirect sources. Employment rate was lower among the non-respondents than the respondents. Non-response was associated with age, social class, previous employment record, and the type of disability; but none of these characteristics were associated with the outcome. Out of the five known independent risk factors for unemployment, only one (incompletion of rehabilitation course) was associated with non-response. The employment rate among the respondents was also assessed according to the delay in response, that is the number of reminders sent to achieve response. The outcome among the late respondents was similar to that among the non-respondents. These data suggest that risk estimates may be biased even when the response rate is greater than 80%, the prevalence of risk factors among non-respondents may not indicate the presence or the degree of non-response bias, but reliable estimates can be obtained from extrapolations of the rates among the respondents according to the delay in response.