To cite the \emph{ irtkd } Zelig model:
 \begin{verse}
 Ben Goodrich and Ying Lu. 2007. "irtkd: K-Dimensional Item Response Model" in Kosuke Imai, Gary King, and Olivia Lau, "Zelig: Everyone's Statistical Software,"\url{http://gking.harvard.edu/zelig} 
\end{verse}