De Boeck, P., Bakker, M., Zwitser, R., Nivard, M., Hofman, A., Tuerlinckx, F. and Partchev, I. 2011. The Estimation of Item Response Models with the lmer Function from the lme4 Package in R. Journal of Statistical Software. 39, 12 (Mar. 2011), 1–28. DOI:https://doi.org/10.18637/jss.v039.i12.