Modeling Within- and Between-Person Differences in the Use of the Middle Category in Likert Scales

When using Likert scales, the inclusion of a middle-category response option poses a challenge for the valid measurement of the psychological attribute of interest. While this middle category is often included to provide respondents with a neutral response option, respondents may in practice also select this category when they do not want to or cannot give an informative response. If one analyzes the response data without considering these two possible uses of the middle response category, measurement may be confounded. In this paper, we propose a response-mixture IRTree model for the analysis of Likert-scale data. This model acknowledges that the middle response category can either be selected as a non-response option (and hence be uninformative for the attribute of interest) or to communicate a neutral position (and hence be informative), and that this choice depends on both person- and item-characteristics. For each observed middle-category response, the probability that it was intended to be informative is modeled, and both the attribute of substantive interest and a non-response tendency are estimated. The performance of the model is evaluated in a simulation study, and the procedure is applied to empirical data from personality psychology.