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Although most scaling formats include an intermediate or neutral response category, little research has been devoted to the analysis of the meaning respondents attach to this category. Results obtained from ten different scales, across two types of item formats (Likert and Polar Choice) support the traditional method of scoring the"?" answer. Although the meaning respondents imply when selecting the "?" is not more ambiguous than the meaning implied in the selection of the other response categories, there does exist evidence for the presence of a variety of uses of the "?" including response styles, ambivalence and indifference. Various suggestions are made for further research and alternate methods of approach to the meaning of the question mark response category.
Dubois et al. (Mon,) studied this question.