title: Examining non-response bias in substance use research--are late respondents proxies for non-respondents?
abstract: BACKGROUND
  Non-response is a major concern among substance use epidemiologists. When differences exist between respondents and non-respondents, survey estimates may be biased. Therefore, researchers have developed time-consuming strategies to convert non-respondents to respondents. The present study examines whether late respondents (converted former non-participants) differ from early respondents, non-consenters or silent refusers (consent givers but non-participants) in a cohort study, and whether non-response bias can be reduced by converting former non-respondents.
  
  
  METHODS
  6099 French- and 5720 German-speaking Swiss 20-year-old males (more than 94% of the source population) completed a short questionnaire on substance use outcomes and socio-demographics, independent of any further participation in a cohort study. Early respondents were those participating in the cohort study after standard recruitment procedures. Late respondents were non-respondents that were converted through individual encouraging telephone contact. Early respondents, non-consenters and silent refusers were compared to late respondents using logistic regressions. Relative non-response biases for early respondents only, for respondents only (early and late) and for consenters (respondents and silent refusers) were also computed.
  
  
  RESULTS
  Late respondents showed generally higher patterns of substance use than did early respondents, but lower patterns than did non-consenters and silent refusers. Converting initial non-respondents to respondents reduced the non-response bias, which might be further reduced if silent refusers were converted to respondents.
  
  
  CONCLUSIONS
  Efforts to convert refusers are effective in reducing non-response bias. However, converted late respondents cannot be seen as proxies of non-respondents, and are at best only indicative of existing response bias due to persistent non-respondents.
authors.affiliation: Alcohol Treatment Centre, Lausanne University Hospital CHUV, Av. Beaumont 21 bis, Pavillon 2, CH-1011 Lausanne, Switzerland. Joseph.Studer@chuv.ch
authors.forename: Joseph
authors.initials: J
authors.lastname: Studer
authors.forename: Stéphanie
authors.initials: S
authors.lastname: Baggio
authors.forename: Meichun
authors.initials: M
authors.lastname: Mohler-Kuo
authors.forename: Petra
authors.initials: P
authors.lastname: Dermota
authors.forename: Jacques
authors.initials: J
authors.lastname: Gaume
authors.forename: Nicolas
authors.initials: N
authors.lastname: Bertholet
authors.forename: Jean-Bernard
authors.initials: JB
authors.lastname: Daeppen
authors.forename: Gerhard
authors.initials: G
authors.lastname: Gmel
mesh_terms: D000428:Alcohol Drinking / Q000523:psychology; D000465:Algorithms; D015982:Bias; D015331:Cohort Studies; D003625:Data Collection; D004522:Educational Status; D015340:Epidemiologic Research Design; D005602:France; D005858:Germany; D006801:Humans; D016015:Logistic Models; D008297:Male; D008385:Marijuana Smoking / Q000523:psychology; D015995:Prevalence; D012307:Risk Factors; D012959:Socioeconomic Factors; D019966:Substance-Related Disorders / Q000523:psychology*; D064424:Tobacco Use / Q000523:psychology; D014505:Urban Population; D055815:Young Adult
keywords: Early respondent; Later respondent; Non-response bias; Substance use; Young men