Middle Response Scale Options are Inappropriate for Ideal Point Scales
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The purpose of this study is to provide a theoretical rationale for the inappropriateness of middle response options on the response scales offered on ideal point scales, and to provide empirical support for this argument to assist ideal point scale development.
The same ideal point scale was administered in three quasi-experimental groups varying only in the response scale offered: the three groups received either a four-, five-, or six-option response scale. An ideal point Item Response Theory model, the Generalized Graded Unfolding Model (GGUM), was fit to the response data, and model-data fit was compared across conditions.
Responses from the four- and six-option conditions were fit well by the GGUM, but responses from the five-option condition were not fit well.
Despite the scale being constructed to follow the tenets of ideal point responding, the GGUM was unable to provide a reasonable probabilistic account of responding when the response scale contained a middle option. The authors find support for the argument that an odd-numbered response scale does not match the principles of ideal point responding, and can actually result in misspecifying the underlying response process.
Although a growing body of research has suggested that attitude and personality measurement is best conceptualized under the assumptions of ideal point responding, little practical advice has been given to researchers or practitioners regarding scale creation. This was the first study to theoretically and empirically assess the response scale on ideal point scales, and offer guidance for constructing ideal point scales.
KeywordsItem Response Theory Measurement Ideal point measurement Survey research
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