title: Effect of numbering of return envelopes on participation, explicit refusals, and bias: experiment and meta-analysis.
abstract: BACKGROUND
  Tracing mail survey responses is useful for the management of reminders but may cause concerns about anonymity among prospective participants. We examined the impact of numbering return envelopes on the participation and the results of a survey on a sensitive topic among hospital staff.
  
  
  METHODS
  In a survey about regrets associated with providing healthcare conducted among hospital-based doctors and nurses, two randomly drawn subsamples were provided numbered (N = 1100) and non-numbered (N = 500) envelopes for the return of completed questionnaires. Participation, explicit refusals, and item responses were compared. We also conducted a meta-analysis of the effect of questionnaire/envelope numbering on participation in health surveys.
  
  
  RESULTS
  The participation rate was lower in the "numbered" group than in the "non-numbered" group (30.3% vs. 35.0%, p = 0.073), the proportion of explicit refusals was higher in the "numbered" group (23.1% vs 17.5%, p = 0.016), and the proportion of those who never returned the questionnaire was similar (46.6% vs 47.5%, p = 0.78). The means of responses differed significantly for 12 of 105 items (11.4%), which did not differ significantly from the expected frequency of type 1 errors, i.e., 5% (permutation test, p = 0.078). The meta-analysis of 7 experimental surveys (including this one) indicated that numbering is associated with a 2.4% decrease in the survey response rate (95% confidence interval 0.3% to 4.4%).
  
  
  CONCLUSIONS
  Numbered return envelopes may reduce the response rate and increase explicit refusals to participate in a sensitive survey. Reduced participation was confirmed by a meta-analysis of randomized health surveys. There was no strong evidence of bias.
authors.affiliation: Division of Clinical Epidemiology, University Hospitals of Geneva, Geneva, Switzerland. Thomas.Perneger@hcuge.ch.
authors.forename: Thomas V
authors.initials: TV
authors.lastname: Perneger
authors.forename: Stéphane
authors.initials: S
authors.lastname: Cullati
authors.forename: Sandrine
authors.initials: S
authors.lastname: Rudaz
authors.forename: Thomas
authors.initials: T
authors.lastname: Agoritsas
authors.forename: Ralph E
authors.initials: RE
authors.lastname: Schmidt
authors.forename: Christophe
authors.initials: C
authors.lastname: Combescure
authors.forename: Delphine S
authors.initials: DS
authors.lastname: Courvoisier
mesh_terms: D000328:Adult; D015982:Bias; D003625:Data Collection / Q000379:methods; D005260:Female; D006306:Health Surveys / Q000379:methods*; D006801:Humans; D008297:Male; D010349:Patient Compliance / Q000706:statistics & numerical data*; D011177:Postal Service; D011446:Prospective Studies; D011897:Random Allocation; D045808:Refusal to Participate / Q000706:statistics & numerical data*; D011795:Surveys and Questionnaires