Rondeau, V., Marzroui, Y. and Gonzalez, J. R. (2012) “frailtypack: An R Package for the Analysis of Correlated Survival Data with Frailty Models Using Penalized Likelihood Estimation or Parametrical Estimation”, Journal of Statistical Software, 47(4), pp. 1–28. doi: 10.18637/jss.v047.i04.