# DART booster¶

XGBoost mostly combines a huge number of regression trees with a small learning rate. In this situation, trees added early are significant and trees added late are unimportant.

Vinayak and Gilad-Bachrach proposed a new method to add dropout techniques from the deep neural net community to boosted trees, and reported better results in some situations.

This is a instruction of new tree booster dart.

## Original paper¶

Rashmi Korlakai Vinayak, Ran Gilad-Bachrach. “DART: Dropouts meet Multiple Additive Regression Trees.” JMLR.

## Features¶

• Drop trees in order to solve the over-fitting.
• Trivial trees (to correct trivial errors) may be prevented.

Because of the randomness introduced in the training, expect the following few differences:

• Training can be slower than gbtree because the random dropout prevents usage of the prediction buffer.
• The early stop might not be stable, due to the randomness.

## How it works¶

• In $$m$$-th training round, suppose $$k$$ trees are selected to be dropped.
• Let $$D = \sum_{i \in \mathbf{K}} F_i$$ be the leaf scores of dropped trees and $$F_m = \eta \tilde{F}_m$$ be the leaf scores of a new tree.
• The objective function is as follows:
$\mathrm{Obj} = \sum_{j=1}^n L \left( y_j, \hat{y}_j^{m-1} - D_j + \tilde{F}_m \right) + \Omega \left( \tilde{F}_m \right).$
• $$D$$ and $$F_m$$ are overshooting, so using scale factor
$\hat{y}_j^m = \sum_{i \not\in \mathbf{K}} F_i + a \left( \sum_{i \in \mathbf{K}} F_i + b F_m \right) .$

## Parameters¶

The booster dart inherits gbtree booster, so it supports all parameters that gbtree does, such as eta, gamma, max_depth etc.

• sample_type: type of sampling algorithm.

• uniform: (default) dropped trees are selected uniformly.
• weighted: dropped trees are selected in proportion to weight.
• normalize_type: type of normalization algorithm.

• tree: (default) New trees have the same weight of each of dropped trees.
$\begin{split}a \left( \sum_{i \in \mathbf{K}} F_i + \frac{1}{k} F_m \right) &= a \left( \sum_{i \in \mathbf{K}} F_i + \frac{\eta}{k} \tilde{F}_m \right) \\ &\sim a \left( 1 + \frac{\eta}{k} \right) D \\ &= a \frac{k + \eta}{k} D = D , \\ &\quad a = \frac{k}{k + \eta}\end{split}$
• forest: New trees have the same weight of sum of dropped trees (forest).
$\begin{split}a \left( \sum_{i \in \mathbf{K}} F_i + F_m \right) &= a \left( \sum_{i \in \mathbf{K}} F_i + \eta \tilde{F}_m \right) \\ &\sim a \left( 1 + \eta \right) D \\ &= a (1 + \eta) D = D , \\ &\quad a = \frac{1}{1 + \eta} .\end{split}$
• rate_drop: dropout rate.

• range: [0.0, 1.0]
• skip_drop: probability of skipping dropout.

• If a dropout is skipped, new trees are added in the same manner as gbtree.
• range: [0.0, 1.0]

## Sample Script¶

import xgboost as xgb
dtrain = xgb.DMatrix('demo/data/agaricus.txt.train')
dtest = xgb.DMatrix('demo/data/agaricus.txt.test')
# specify parameters via map
param = {'booster': 'dart',
'max_depth': 5, 'learning_rate': 0.1,
'objective': 'binary:logistic', 'silent': True,
'sample_type': 'uniform',
'normalize_type': 'tree',
'rate_drop': 0.1,
'skip_drop': 0.5}
num_round = 50
bst = xgb.train(param, dtrain, num_round)
# make prediction
# ntree_limit must not be 0
preds = bst.predict(dtest, ntree_limit=num_round)


Note

Specify ntree_limit when predicting with test sets

By default, bst.predict() will perform dropouts on trees. To obtain correct results on test sets, disable dropouts by specifying a nonzero value for ntree_limit.