Figure 1

Rasch model category probability curves of a question with four response categories (1, not at all; 2, a little; 3, quite a bit; and 4, a lot). The x-axis represents the attribute in logits. The y-axis represents the probability of a response category being selected. The curves represent the likelihood that a respondent with a particular amount of the latent trait will select a category: illustration of the concepts of scale range (−3 to +3, i.e. 6 logits in this example), 3 thresholds for 4 categories and evenness of categories (category width, 3 logits each; standard deviation of the width, 0).