  High-cardinality categorical variables are variables for which the number of
different levels is large relative to the sample size of a data set, or in
other words, there are few data points per level. Machine learning methods can
have difficulties with high-cardinality variables. In this article, we
empirically compare several versions of two of the most successful machine
learning methods, tree-boosting and deep neural networks, and linear mixed
effects models using multiple tabular data sets with high-cardinality
categorical variables. We find that, first, machine learning models with random
effects have higher prediction accuracy than their classical counterparts
without random effects, and, second, tree-boosting with random effects
outperforms deep neural networks with random effects.
