Investigating the functioning of a middle category by means of a mixed-measurement model

Researchers have studied whether there are classes of people who differ systematically in the way they respond to polytomous ordered scales with a middle category such as ?. The mixed-partial credit model was fitted to a number of scales of a personality questionnaire. Most of the scales fit better with the use of 2 latent subpopulations. The most consistent difference among the latent classes was related to the functioning of the middle response category. For most of the examinees, the probability of choosing the middle category was very close to zero, but a nonnegligible percentage of people selected this category with much higher probability. The total scores from the 2 subpopulations were incommensurate. Some personality factors contributed to explaining class membership.