Software

Summary

Several methods for assessing differential item functioning, such as Raju's Differential Functioning of Items and Tests (DFIT), require estimates of the covariance matrix of item parameters. Until recently, the full covariance matrix was not readily available from common IRT estimation software. A Fortran program was created to compute the item parameter covariances for the Graded Response Models, a common IRT model for Likert-type response scales.

Details

The program reads an IRT parameter file and computes covariance of item parameters for the Graded Response Model using the approach described in

Morris, S. B., Fortmann-Johnson, K., and Oshima, T.C. (2007). An Evaluation of the Item Parameter Replication Method for DFIT Analysis of Polytomous Items. Paper presented at the annual conference of the National Council of Measurement in Education, Chicago, IL.

The dowload includes a Fortran program, instruction manual and sample input files.

This software is copyright (c) 2009 by Dr. Scott B. Morris. It available free for research or educational purposes. For written permission to use it for any other purpose, please contact Scott B. Morris, Department of Psychology, Illinois Institute of Technology, 3424 S. State St., Chicago, IL 60616. Email: scott.morris@iit.edu