title: The effect of multiple reminders on response patterns in a Danish health survey.
abstract: BACKGROUND
  Reminders are routinely applied in surveys to increase response rates and reduce the possibility of bias. This study examines the effect of multiple reminders on the response rate, non-response bias, prevalence estimates and exposure-outcome relations in a national self-administered health survey.
  
  
  METHODS
  Data derive from the Danish National Health Survey 2010, in which 298,550 individuals (16 years of age or older) were invited to participate in a cross-sectional survey using a mixed-mode approach (paper and web questionnaires). At least two reminders were sent to non-respondents, and 177,639 individuals completed the questionnaire (59.5%). Response patterns were compared between four groups of individuals (first mailing respondents, second mailing respondents, third mailing respondents and non-respondents).
  
  
  RESULTS
  Multiple reminders led to an increase in response rate from 36.7 to 59.5%; however, the inclusion of second and third mailing respondents did not change the overall characteristics of respondents compared with non-respondents. Furthermore, only small changes in prevalence estimates and exposure-outcome relationships were observed when including second and third mailing respondents compared with only first mailing respondents.
  
  
  CONCLUSIONS
  Multiple reminders were an effective way to increase the response rate in a national Danish health survey. However, when differences do exist between respondents and non-respondents, the results suggest that second and third mailings are unlikely to eliminate these differences. Overall, multiple reminders seemed to have only minor effect on response patterns and study conclusions in the present study.
authors.affiliation: 1 National Institute of Public Health, University of Southern Denmark, Copenhagen, Denmark ach@niph.dk.
authors.forename: Anne I
authors.initials: AI
authors.lastname: Christensen
authors.affiliation: 1 National Institute of Public Health, University of Southern Denmark, Copenhagen, Denmark.
authors.forename: Ola
authors.initials: O
authors.lastname: Ekholm
authors.affiliation: 2 Region of Southern Denmark, Vejle, Denmark.
authors.forename: Peter L
authors.initials: PL
authors.lastname: Kristensen
authors.affiliation: 3 Centre for Public Health, Central Denmark Region, Aarhus, Denmark.
authors.forename: Finn B
authors.initials: FB
authors.lastname: Larsen
authors.affiliation: 4 North Denmark Region, Aalborg, Denmark.
authors.forename: Anker L
authors.initials: AL
authors.lastname: Vinding
authors.affiliation: 5 Research Centre for Prevention and Health, The Capital Region of Denmark, Glostrup, Denmark 6 Department of Health Science and Technology, Aalborg University, Aalborg, Denmark.
authors.forename: Charlotte
authors.initials: C
authors.lastname: Glümer
authors.affiliation: 1 National Institute of Public Health, University of Southern Denmark, Copenhagen, Denmark.
authors.forename: Knud
authors.initials: K
authors.lastname: Juel
mesh_terms: D000293:Adolescent; D000328:Adult; D000368:Aged; D000369:Aged, 80 and over; D003430:Cross-Sectional Studies; D003463:Cues; D003718:Denmark; D005260:Female; D006306:Health Surveys / Q000379:methods* / Q000706:statistics & numerical data*; D006801:Humans; D008297:Male; D008875:Middle Aged; D011795:Surveys and Questionnaires; D055815:Young Adult