This article summarizes the findings of an experiment to evaluate the effect of "Don't Know" (DK) boxes in a mail questionnaire consisting of factual questions. In a split-panel test, half of the respondents received a questionnaire with DK boxes and the other half received a questionnaire without DK boxes. The overall response rates were the same for the two versions. The questionnaire version without DK boxes obtained an appreciably higher rate of substantive responses for many items. However, there were few large substantive changes in response distributions between the two versions, and there was no appreciable difference in the response error rates.In the development of each question in a survey questionnaire, the survey designer must decide, in addition to question wording, what response categories are most appropriate. Usually, closed response categories are preferred to facilitate coding and analysis.
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