title: Non-response bias in a web-based health behaviour survey of New Zealand tertiary students.
abstract: OBJECTIVE
  There has been little investigation of non-response bias in web-based health surveys. We hypothesised that non-respondents have a higher prevalence of risk behaviours than respondents.
  
  
  METHODS
  In 2005, random samples of students aged 17-25 years from 12 New Zealand tertiary institutions (n=7130) were invited to complete a web-based health behaviour survey, with three e-mail reminders. Early respondents (before 2nd reminder) were compared with late respondents (after 2nd reminder). Late respondents served as a proxy for non-respondents.
  
  
  RESULTS
  2607 students (37%) responded early, 676 (9%) responded late, and 3847 (54%) did not respond. There were differences between early and late respondents in high school binge drinking (38% vs 47%, p=0.002) and non-compliance with physical activity guidelines (12% vs 18%, p=0.004). Differences in overweight/obesity (26% vs 31%, p=0.058), smoking (18% vs 22%, p=0.091) and non-compliance with dietary guidelines (76% vs 77%, p=0.651) were non-significant but point estimates were in the expected direction. Estimated bias in prevalence of risk behaviours was an absolute difference of 1-4% and a relative difference of 0-21%.
  
  
  CONCLUSIONS
  Respondents whose participation was hardest to elicit reported more risk behaviour. Assuming non-respondents' behaviour is similar or more extreme than that of late respondents, prevalence will have been substantially underestimated.
authors.affiliation: School of Medicine and Public Health, University of Newcastle, Callghan, Australia. kypros.kypri@newcastle.edu.au
authors.forename: Kypros
authors.initials: K
authors.lastname: Kypri
authors.forename: Ari
authors.initials: A
authors.lastname: Samaranayaka
authors.forename: Jennie
authors.initials: J
authors.lastname: Connor
authors.forename: John D
authors.initials: JD
authors.lastname: Langley
authors.forename: Brett
authors.initials: B
authors.lastname: Maclennan
mesh_terms: D000293:Adolescent; D000428:Alcohol Drinking / Q000453:epidemiology; D015982:Bias; D015444:Exercise; D005260:Female; D015438:Health Behavior; D006306:Health Surveys; D006801:Humans; D020407:Internet; D008297:Male; D009520:New Zealand / Q000453:epidemiology; D050177:Overweight / Q000453:epidemiology; D015995:Prevalence; D012309:Risk-Taking; D057566:Self Report; D013334:Students; D055815:Young Adult