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 nonrespondents. These data suggest that (a) risk estimates may be biased even when the response rate is greater than 80%, (b) the prevalence of risk factors among non-respondents may not indicate the presence or the degree of non-response bias, but (c) reliable estimates can be obtained from extrapolations of the rates among the respondents according to the delay in response.