To cite the \emph{ gamma.mixed } Zelig model:
 \begin{verse}
 Delia Bailey and Ferdinand Alimadhi. 2007. "gamma.mixed: Mixed effects gamma model" in Kosuke Imai, Gary King, and Olivia Lau, "Zelig: Everyone's Statistical Software,"\url{http://gking.harvard.edu/zelig} 
\end{verse}