Abstract
BackgroundFew longitudinal studies of disaster cohorts have assessed both non-response bias in prevalence estimates of health outcomes and in the estimates of associations between health outcomes and disaster exposures. We examined the factors associated with non-response and the possible non-response bias in prevalence estimates and association estimates in a longitudinal study of World Trade Center (WTC) terrorist attack survivors.MethodsIn 2003–04, 71,434 enrollees completed the WTC Health Registry wave 1 health survey. This study is limited to 67,670 adults who were eligible for both wave 2 and wave 3 surveys in 2006–07 and 2011–12. We first compared the characteristics between wave 3 participants (wave 3 drop-ins and three-wave participants) and non-participants (wave 3 drop-outs and wave 1 only participants). We then examined potential non-response bias in prevalence estimates and in exposure-outcome association estimates by comparing one-time non-participants (wave 3 drop-ins and drop-outs) at the two follow-up surveys with three-wave participants.ResultsCompared to wave 3 participants, non-participants were younger, more likely to be male, non-White, non-self enrolled, non-rescue or recovery worker, have lower household income, and less than post-graduate education. Enrollees’ wave 1 health status had little association with their wave 3 participation. None of the disaster exposure measures measured at wave 1 was associated with wave 3 non-participation. Wave 3 drop-outs and drop-ins (those who participated in only one of the two follow-up surveys) reported somewhat poorer health outcomes than the three-wave participants. For example, compared to three-wave participants, wave 3 drop-outs had a 1.4 times higher odds of reporting poor or fair health at wave 2 (95% CI 1.3-1.4). However, the associations between disaster exposures and health outcomes were not different significantly among wave 3 drop-outs/drop-ins as compared to three-wave participants.ConclusionOur results show that, despite a downward bias in prevalence estimates of health outcomes, attrition from the WTC Health Registry follow-up studies does not lead to serious bias in associations between 9/11 disaster exposures and key health outcomes. These findings provide insight into the impact of non-response on associations between disaster exposures and health outcomes reported in longitudinal studies.
Highlights
Few longitudinal studies of disaster cohorts have assessed both non-response bias in prevalence estimates of health outcomes and in the estimates of associations between health outcomes and disaster exposures
Regarding health status (Table 2), enrollees who did not participate in wave 3 were slightly more likely to have probable posttraumatic stress disorder (PTSD) at wave 1 (17.9% as compared to 15.2% of non-participants, adjusted odds ratios (AOR) =1.1, 95% confidence intervals (CI): 1.1-1.2) and slightly less likely to have new or worsening respiratory symptoms since 9/11 (65.9% vs. 68.0%)
Non-response bias has been a concern since the inception of the World Trade Center (WTC) Health Registry [42]
Summary
Few longitudinal studies of disaster cohorts have assessed both non-response bias in prevalence estimates of health outcomes and in the estimates of associations between health outcomes and disaster exposures. The assessment of long-term health outcomes among individuals exposed to the September 11, 2001, terrorist attack on the World Trade Center (WTC) in New York City poses epidemiological challenges that may more generally be encountered following large scale natural or man-made disasters. One such challenge is to identify and quantify long-term health outcomes in a cohort consisting of a heterogeneous mixture of distinct survivor groups. To better understand the long-term health effects of the 9/11 disaster, and to better assess the postdisaster health care needs of survivors, a timely evaluation of selective participation in the follow-up studies and non-response bias in this unique disaster cohort is needed
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