In Search of the Optimal Number of Response Categories in a Rating Scale

Abstract

Likert-type rating scales are still the most widely used method when measuring psychoeducational constructs. The present study investigates a long-standing issue of identifying the optimal number of response categories. A special emphasis is given to categorical data, which were generated by the Item Response Theory (IRT) Graded-Response Modeling (GRM). Along with number of categories (from 2 to 6), two scale characteristics of scale length (n = 5, 10, and 20 items) and item discrimination (high/medium/low) were examined. Results of this study show that there was virtually no difference in psychometric properties of the scales using 4, 5, or 6 categories. Most deteriorating change was observed when the number of response categories reduced from 3 to 2 points in all six psychometric measures. Small moderating effects by scale length and item discrimination seem to be present, that is, a slightly larger impact on the psychometric properties by changing the number of response categories in a shorter and/or highly discriminating scale. This study concludes with the suggestion that a caution should be made if a scale has only 2 response categories but that limitation may be overcome by manipulating other scale features, namely, scale length or item discrimination.