Helper functions and example data sets to facilitate the estimation
of tree-based item Response models of the GLMM family with function
glmer from the
Ivailo Partchev <firstname.lastname@example.org>
De Boeck, P. and Partchev, I. (2012). IRTrees: Tree-Based Item Response Models of the GLMM Family, Journal of Statistical Software – Code Snippets, 48(1), 1–28. http://www.jstatsoft.org/v48/c01/.