Random Tree (RapidMiner Studio Core)

Synopsis

This operator learns a decision tree. This operator uses only a random subset of attributes for each split.

Description

The Random Tree operator works exactly like the Decision Tree operator with one exception: for each split only a random subset of attributes is available. It is recommended that you study the documentation of the Decision Tree operator for basic understanding of decision trees.

This operator learns decision trees from both nominal and numerical data. Decision trees are powerful classification methods which can be easily understood. The Random Tree operator works similar to Quinlan's C4.5 or CART but it selects a random subset of attributes before it is applied. The size of the subset is specified by the subset ratio parameter.

Representation of the data as Tree has the advantage compared with other approaches of being meaningful and easy to interpret. The goal is to create a classification model that predicts the value of the label based on several input attributes of the ExampleSet. Each interior node of tree corresponds to one of the input attributes. The number of edges of an interior node is equal to the number of possible values of the corresponding input attribute. Each leaf node represents a value of the label given the values of the input attributes represented by the path from the root to the leaf. This description can be easily understood by studying the Example Process of the Decision Tree operator.

Pruning is a technique in which leaf nodes that do not add to the discriminative power of the decision tree are removed. This is done to convert an over-specific or over-fitted tree to a more general form in order to enhance its predictive power on unseen datasets. Pre-pruning is a type of pruning performed parallel to the tree creation process. Post-pruning, on the other hand, is done after the tree creation process is complete.

Differentiation

Decision Tree

The Random Tree operator works exactly like the Decision Tree operator with one exception: for each split only a random subset of attributes is available.

Input

training set (Data Table)
This input port expects an ExampleSet. It is the output of the Retrieve operator in the attached Example Process. The output of other operators can also be used as input.

Output

model (Decision Tree)
The Random Tree is delivered from this output port. This classification model can now be applied on unseen data sets for the prediction of the label attribute.
example set (Data Table)
The ExampleSet that was given as input is passed without changing to the output through this port. This is usually used to reuse the same ExampleSet in further operators or to view the ExampleSet in the Results Workspace.

Parameters

criterionThis parameter selects the criterion on which attributes will be selected for splitting. It can have one of the following values:
- information_gain: The entropy of all the attributes is calculated. The attribute with minimum entropy is selected for split. This method has a bias towards selecting attributes with a large number of values.
- gain_ratio: It is a variant of information gain. It adjusts the information gain for each attribute to allow the breadth and uniformity of the attribute values.
- gini_index: This is a measure of impurity of an ExampleSet. Splitting on a chosen attribute gives a reduction in the average gini index of the resulting subsets.
- accuracy: Such an attribute is selected for split that maximizes the accuracy of the whole Tree.
Range: selection
minimal_size_for_splitThe size of a node in a Tree is the number of examples in its subset. The size of the root node is equal to the total number of examples in the ExampleSet. Only those nodes are split whose size is greater than or equal to the minimal size for split parameter. Range: integer
minimal_leaf_sizeThe size of a leaf node in a Tree is the number of examples in its subset. The tree is generated in such a way that every leaf node subset has at least the minimal leaf size number of instances. Range: integer
minimal_gainThe gain of a node is calculated before splitting it. The node is split if its Gain is greater than the minimal gain. Higher value of minimal gain results in fewer splits and thus a smaller tree. A too high value will completely prevent splitting and a tree with a single node is generated. Range: real
maximal_depthThe depth of a tree varies depending upon size and nature of the ExampleSet. This parameter is used to restrict the size of the Tree. The tree generation process is not continued when the tree depth is equal to the maximal depth. If its value is set to '-1', the maximal depth parameter puts no bound on the depth of the tree, a tree of maximum depth is generated. If its value is set to '1', a Tree with a single node is generated. Range: integer
confidenceThis parameter specifies the confidence level used for the pessimistic error calculation of pruning. Range: real
number_of_prepruning_alternativesAs prepruning runs parallel to the tree generation process, it may prevent splitting at certain nodes when splitting at that node does not add to the discriminative power of the entire tree. In such a case alternative nodes are tried for splitting. This parameter adjusts the number of alternative nodes tried for splitting when split is prevented by prepruning at a certain node. Range: integer
no_prepruningBy default the Tree is generated with prepruning. Setting this parameter to true disables the prepruning and delivers a tree without any prepruning. Range: boolean
no_pruningBy default the Tree is generated with pruning. Setting this parameter to true disables the pruning and delivers an unpruned Tree. Range: boolean
guess_subset_ratioThis parameter specifies if the subset ratio should be guessed or not. If set to true, log(m) + 1 features are used as subset, otherwise a ratio has to be specified through the subset ratio parameter. Range: boolean
subset_ratioThis parameter specifies the subset ratio of randomly chosen attributes. Range: real
use_local_random_seedThis parameter indicates if a local random seed should be used for randomization. Using the same value of the local random seed will produce the same randomization. Range: boolean
local_random_seedThis parameter specifies the local random seed. This parameter is only available if the use local random seed parameter is set to true. Range: integer

Tutorial Processes

Introduction to the Random Tree operator

The 'Iris' data set is loaded using the Retrieve operator. A breakpoint is inserted here so that you can have a look at the ExampleSet. The Random Tree operator is applied on this ExampleSet with default values of all parameters. The resultant tree is connected to the result port of the process and it can be seen in the Results Workspace.

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