Choi, S. W., L. E. Gibbons, and P. K. Crane. “Lordif: An R Package for Detecting Differential Item Functioning Using Iterative Hybrid Ordinal Logistic Regression/Item Response Theory and Monte Carlo Simulations”. Journal of Statistical Software, vol. 39, no. 8, Mar. 2011, pp. 1-30, doi:10.18637/jss.v039.i08.