The IRT model most appropriate for binary data such as substance abuse symptoms is the 2-parameter model 10, 11. Under this model, 2 parameters are estimated for each symptom: severity (also known as “item difficulty” in the IRT literature) and discrimination. Symptom severity is defined as the severity point on the continuum of the latent AUD trait at which the probability of endorsing the symptom is 50%. Symptom severity tells us at what severity of disorder the symptom is most informative (i.e., is it most useful in scaling those with very serious or less serious AUDs?). Symptom discrimination represents the symptom’s ability to discriminate between persons with severity of disorder above and below the symptom’s threshold. The discrimination and severity parameters are similar to factor loadings and threshold parameters in a dichotomous factor analysis 31.
