  This paper is a comment on the survey paper by Biau and Scornet (2016) about
random forests. We focus on the problem of quantifying the impact of each
ingredient of random forests on their performance. We show that such a
quantification is possible for a simple pure forest , leading to conclusions
that could apply more generally. Then, we consider "hold-out" random forests,
which are a good middle point between "toy" pure forests and Breiman's original
random forests.
