Objectives: In self-completed surveys, anonymous questionnaires are sometimes numbered so as to avoid sending reminders to initial nonrespondents. This number may be perceived as a threat to confidentiality by some respondents, which may reduce the response rate, or cause social desirability bias. In this study, we evaluated whether using nonnumbered vs. numbered questionnaires influenced the response rate and the response content.
Study design and setting: During a patient safety culture survey, we randomized participants into two groups: one received an anonymous nonnumbered questionnaire and the other a numbered questionnaire. We compared the survey response rates and distributions of the responses for the 42-questionnaire items across the two groups.
Results: Response rates were similar in the two groups (nonnumbered, 75.2%; numbered, 72.8%; difference, 2.4%; P=0.28). Five of the 42 questions had statistically significant differences in distributions, but these differences were small. Unexpectedly, in all five instances, the patient safety culture ratings were more favorable in the nonnumbered group.
Conclusion: Numbering of mailed questionnaires had no impact on the response rate. Numbering influenced significantly the response content of several items, but these differences were small and ran against the hypothesis of social desirability bias.
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