@article{JMLR:v19:16-474,
  author  = {Timothy C. Au},
  title   = {Random Forests, Decision Trees, and Categorical Predictors: The ``Absent Levels'' Problem},
  journal = {Journal of Machine Learning Research},
  year    = {2018},
  volume  = {19},
  number  = {45},
  pages   = {1--30},
  url     = {http://jmlr.org/papers/v19/16-474.html}
}