title: Evaluation of non-response bias in a cohort study of World Trade Center terrorist attack survivors.
abstract: BACKGROUND
  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. 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.
  
  
  METHODS
  In 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.
  
  
  RESULTS
  Compared 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.
  
  
  CONCLUSIONS
  Our 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.
authors.affiliation: New York City Department of Health and Mental Hygiene, Long Island City, NY, USA. syu@health.nyc.gov.
authors.forename: Shengchao
authors.initials: S
authors.lastname: Yu
authors.affiliation: New York City Department of Health and Mental Hygiene, Long Island City, NY, USA. rbrackbi@health.nyc.gov.
authors.forename: Robert M
authors.initials: RM
authors.lastname: Brackbill
authors.affiliation: New York City Department of Health and Mental Hygiene, Long Island City, NY, USA. sstellma@health.nyc.gov.|Department of Epidemiology, Mailman School of Public Health, Columbia University, New York, NY, USA. sstellma@health.nyc.gov.
authors.forename: Steven D
authors.initials: SD
authors.lastname: Stellman
authors.affiliation: New York City Department of Health and Mental Hygiene, Long Island City, NY, USA. sghuman@health.nyc.gov.
authors.forename: Sharon
authors.initials: S
authors.lastname: Ghuman
authors.affiliation: New York City Department of Health and Mental Hygiene, Long Island City, NY, USA. mfarfel@health.nyc.gov.
authors.forename: Mark R
authors.initials: MR
authors.lastname: Farfel
mesh_terms: D000293:Adolescent; D000328:Adult; D015982:Bias; D002648:Child; D060731:Emergency Responders / Q000523:psychology; D005260:Female; D006304:Health Status; D006306:Health Surveys / Q000706:statistics & numerical data*; D006801:Humans; D019570:Inhalation Exposure / Q000009:adverse effects; D008137:Longitudinal Studies; D008297:Male; D008875:Middle Aged; D009518:New York / Q000453:epidemiology; D012042:Registries; D012130:Respiratory Hypersensitivity / Q000453:epidemiology / Q000209:etiology / Q000503:physiopathology / Q000523:psychology*; D047851:September 11 Terrorist Attacks / Q000523:psychology*; D013313:Stress Disorders, Post-Traumatic / Q000453:epidemiology / Q000503:physiopathology / Q000523:psychology*; D011795:Surveys and Questionnaires; D017741:Survivors / Q000523:psychology; D055815:Young Adult