Open Access Research article

Effect of numbering of return envelopes on participation, explicit refusals, and bias: experiment and meta-analysis

Thomas V Perneger12*, Stéphane Cullati12, Sandrine Rudaz1, Thomas Agoritsas123, Ralph E Schmidt4, Christophe Combescure12 and Delphine S Courvoisier12

Author Affiliations

1 Division of Clinical Epidemiology, University Hospitals of Geneva, Geneva, Switzerland

2 Faculty of Medicine, University of Geneva, Geneva, Switzerland

3 Department of Clinical Epidemiology and Biostatistics, McMaster University, Hamilton, Canada

4 Swiss Center for Affective Sciences, University of Geneva, Geneva, Switzerland

For all author emails, please log on.

BMC Medical Research Methodology 2014, 14:6  doi:10.1186/1471-2288-14-6

Published: 15 January 2014



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.


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.


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%).


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.

Health surveys; Survey participation; Survey numbering; Bias; Sensitive topic; Meta-analysis