October 25th, 2017
(written by lawrence krubner, however indented passages are often quotes). You can contact lawrence at: firstname.lastname@example.org
It is important that the weak learners have skill but remain weak.
There are a number of ways that the trees can be constrained.
A good general heuristic is that the more constrained tree creation is, the more trees you will need in the model, and the reverse, where less constrained individual trees, the fewer trees that will be required.
Below are some constraints that can be imposed on the construction of decision trees:
Number of trees, generally adding more trees to the model can be very slow to overfit. The advice is to keep adding trees until no further improvement is observed.
Tree depth, deeper trees are more complex trees and shorter trees are preferred. Generally, better results are seen with 4-8 levels.
Number of nodes or number of leaves, like depth, this can constrain the size of the tree, but is not constrained to a symmetrical structure if other constraints are used.
Number of observations per split imposes a minimum constraint on the amount of training data at a training node before a split can be considered
Minimim improvement to loss is a constraint on the improvement of any split added to a tree.