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Using simulated data, MUL TILOG and P ARSCALE were compared on their recovery of item and trait parameters under the graded response and generalized partial credit item response theory models. The shape of the latent population distribution (normal, skewed, or uniform) and the sample size (250 or 500) were varied. Parameter estimates were essentially unbiased under all conditions, and the root mean square error was similar for both software packages. The choice between these packages can therefore be based on considerations other than the accuracy of parameter estimation.