The key finding of this article is that non-response to clinical follow-up was associated with the same doubling of the mortality risk as non-response to baseline. Non-response to one or more follow-up surveys was associated with a 1.4-fold excess risk among those who responded to their last possible survey, but a 2.3-fold excess risk among those who missed their last possible survey. We also found that excess mortality risk is negligible among partial responders to questionnaire-only surveys, but similar to that for non-responders for participants who miss a medical examination but respond partially through questionnaire completion. In addition, the study examined non-response by socioeconomic position and found no evidence that socioeconomic position modifies the association between non-response and mortality. Our findings that non-responders to baseline had a mortality hazard double that for responders is in line with findings from previous studies.2,5,7–14,20–26
Although a number of studies have reported somewhat higher levels of non-response in low socioeconomic groups,5,6,23,27–33
few studies have examined whether the effects of non-response differ across the socioeconomic strata.29,30,33
In the present study, mortality risks among non-responders were little attenuated by adjustment for socioeconomic position measured by employment grade. Similarly, analyses within grade provided no evidence of differences in the associations between non-response and mortality. Recent findings from the population-based FINRISK surveys similarly observed little attenuation of the association between non-response to baseline and mortality on adjustment for socioeconomic position, and a 2-fold higher mortality risk in non-responders to baseline in every socioeconomic category.14
Studies that have examined morbidity have similarly provided little evidence that the bias introduced by non-response to baseline differs across socioeconomic strata.29,33
It is implausible that there is a direct association between non-response and mortality, rather it is an association driven by a variety of common causes, for example ill-health, caring and accessibility. Some of these, such as measureable ill-health at baseline, were available for participants in the Whitehall II study, but not for non-responders to baseline, and many others were not measured. Consequently, we have chosen to present our analyses adjusted only for age, sex and grade, which were available for everyone included in the analyses. Participants in the Whitehall II study were white-collar civil servants at study entry and covered a wide range of grades with a salary difference of >10-fold between the top and bottom of the hierarchy. In this respect they are representative of the majority of employees in Western countries. Nonetheless, this work needs to be repeated in general population samples as the generalizability of our findings will be limited by the healthy worker effect at baseline, which may produce an underestimation of associations between non-response and mortality.
Our findings from a study with repeat data collections show the excess mortality associated with non-response to previous follow-up surveys is ~1.4-fold if the most recent possible survey is completed, but >2-fold if it is missed. When collecting questionnaire data only, the health-related selection introduced by the inclusion of partial responders who only complete a short telephone interview is similar to that for those who complete a full questionnaire. This means their inclusion in analyses, while increasing the power of the study, will not reduce the bias associated with complete non-response to follow-up at that phase. However, when data collection includes both a medical and a questionnaire, partial responders who miss the medical examination are similar in health status to those who are complete non-responders. Inclusion in analyses of questionnaire or telephone interview data for these partial responders would thus both increase power and reduce the health-selection bias associated with non-response to follow-up. Our findings regarding partial responders are plausible. Attendance at a medical examination requires much more time and effort than completing a questionnaire or short-form telephone interview at home. Thus, non-response to surveys that include a medical examination outside the home is likely to capture a wider range of the unmeasured common causes of non-response and mortality. Additionally, any association between non-response and mortality may be strengthened further if non-responders miss a diagnosis or treatment that might have been brought to attention by the medical examination.
With respect to studies designed to examine inequalities in health, good news is provided by evidence that socioeconomic position does not modify the association between non-response and mortality.