c Parameter

In item response theory (IRT), the c parameter is the lower asymptote of an item characteristic curve (ICC). It is used in the three-parameter logistic (3PL) IRT model, and it is often referred to as the pseudo-guessing parameter. In IRT, the probability that an examinee will make a particular response to a test item is modeled on the examinee’s standing on the trait that the test measures. As an example, for a mathematics achievement test, IRT can model the probability that an examinee will earn a particular score on a mathematics test item based on the examinee’s achievement in math. The ICC visually maps the relationship between an examinee’s ability, usually denoted by θ, and the examinee’s probability of making a particular item response (Figure ...