As a preliminary to the IRT analyses, factor analyses were conducted in a confirmatory factor analytic context using Mplus software (Muthén and Muthén, 2004) to test the IRT assumption of unidimensionality and to determine goodness of fit of a one-factor model. The parameters of the IRT model are most interpretable when the criteria reflect a single unitary dimension (Stout, 1987; Downing, 2003; Bolt et al., 2004; Krueger et al., 2004).
