The a and b parameters were plotted graphically as criterion response curves (CRCs). In these plots the b parameter represents the criterion’s location along the latent continuum (located on the horizontal axis). The b parameter (severity) is the point on the latent continuum where there is a 50% chance of the criterion being present. The b parameter shifts the CRC from left to right as the criterion becomes more severe. Criterion severity is an indication of the degree of criterion severity needed for a particular criterion to be present. The a or discrimination parameter indicates how steep the slope of the CRC is at its steepest point. Although not often done in prior research in this domain, the present study examined the question of whether the two-parameter IRT model indeed provided a better fit to the data than the one-parameter IRT model containing only the b parameter. This comparison was conducted for models with and without the cannabis use criterion, using the Bayesian information function (BIC) as the fit index.
