Github user jkbradley commented on a diff in the pull request:
https://github.com/apache/spark/pull/3461#discussion_r21113959
--- Diff: docs/mllib-gbt.md ---
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+---
+layout: global
+title: Gradient-Boosted Trees - MLlib
+displayTitle: <a href="mllib-guide.html">MLlib</a> - Gradient-Boosted Trees
+---
+
+* Table of contents
+{:toc}
+
+[Gradient-Boosted Trees
(GBTs)](http://en.wikipedia.org/wiki/Gradient_boosting)
+are ensembles of [decision trees](mllib-decision-tree.html).
+GBTs iteratively train decision trees in order to minimize a loss function.
+Like decision trees, GBTs handle categorical features,
+extend to the multiclass classification setting, do not require
+feature scaling, and are able to capture non-linearities and feature
interactions.
+
+MLlib supports GBTs for binary classification and for regression,
+using both continuous and categorical features.
+MLlib implements GBTs using the existing [decision
tree](mllib-decision-tree.html) implementation. Please see the decision tree
guide for more information on trees.
+
+*Note*: GBTs do not yet support multiclass classification. For multiclass
problems, please use
+[decision trees](mllib-decision-tree.html) or [Random
Forests](mllib-random-forest.html).
+
+## Basic algorithm
+
+Gradient boosting iteratively trains a sequence of decision trees.
+On each iteration, the algorithm uses the current ensemble to predict the
label of each training instance and then compares the prediction with the true
label. The dataset is re-labeled to put more weight on training instances with
poor predictions. Thus, in the next iteration, the decision tree will help
correct for previous mistakes.
+
+The specific weight mechanism is defined by a loss function (discussed
below). With each iteration, GBTs further reduce this loss function on the
training data.
+
+### Comparison with Random Forests
+
+Both GBTs and [Random Forests](mllib-random-forest.html) are algorithms
for learning ensembles of trees, but the training processes are different.
There are several practical trade-offs:
+
+ * GBTs may be able to achieve the same accuracy using fewer trees, so the
model produced may be smaller (faster for test time prediction).
+ * GBTs train one tree at a time, so they can take longer to train than
random forests. Random Forests can train multiple trees in parallel.
+ * On the other hand, it is often reasonable to use smaller trees with
GBTs than with Random Forests, and training smaller trees takes less time.
+ * Random Forests can be less prone to overfitting. Training more trees
in a Random Forest reduces the likelihood of overfitting, but training more
trees with GBTs increases the likelihood of overfitting.
+
+In short, both algorithms can be effective. GBTs may be more useful if
test time prediction speed is important. Random Forests are arguably more
successful in industry.
--- End diff --
I'll say less : )
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