# ID3 (RapidMiner Studio Core)

## Synopsis

This operator learns an unpruned Decision Tree from nominal data for classification. This decision tree learner works similar to Quinlan's ID3.

## Description

ID3 (Iterative Dichotomiser 3) is an algorithm used to generate a decision tree invented by Ross Quinlan. ID3 is the precursor to the C4.5 algorithm. Very simply, ID3 builds a decision tree from a fixed set of examples. The resulting tree is used to classify future samples. The examples of the given ExampleSet have several attributes and every example belongs to a class (like yes or no). The leaf nodes of the decision tree contain the class name whereas a non-leaf node is a decision node. The decision node is an attribute test with each branch (to another decision tree) being a possible value of the attribute. ID3 uses feature selection heuristic to help it decide which attribute goes into a decision node. The required heuristic can be selected by the criterion parameter.

The ID3 algorithm can be summarized as follows: Take all unused attributes and calculate their selection criterion (e.g. information gain) Choose the attribute for which the selection criterion has the best value (e.g. minimum entropy or maximum information gain) Make node containing that attribute

ID3 searches through the attributes of the training instances and extracts the attribute that best separates the given examples. If the attribute perfectly classifies the training sets then ID3 stops; otherwise it recursively operates on the n (where n = number of possible values of an attribute) partitioned subsets to get their best attribute. The algorithm uses a greedy search, meaning it picks the best attribute and never looks back to reconsider earlier choices.

Some major benefits of ID3 are:

• Understandable prediction rules are created from the training data.
• Builds a short tree in relatively small time.
• It only needs to test enough attributes until all data is classified.
• Finding leaf nodes enables test data to be pruned, reducing the number of tests.

ID3 may have some disadvantages in some cases e.g.

• Data may be over-fitted or over-classified, if a small sample is tested.
• Only one attribute at a time is tested for making a decision.

## Input

• training set (Data Table)

This input port expects an ExampleSet. It is the output of the Generate Nominal Data operator in the attached Example Process. This operator cannot handle numerical attributes. The output of other operators can also be used as input.

## Output

• model (Decision Tree)

The Decision 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 specifies 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 a split that maximizes the accuracy of the whole Tree.
Range: selection
• minimal_size_for_splitThe size of a node 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 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

## Tutorial Processes

### Getting started with ID3

To understand the basic terminology of trees, please study the Example Process of the Decision Tree operator.

The Generate Nominal Data operator is used for generating an ExampleSet with nominal attributes. It should be kept in mind that the ID3 operator cannot handle numerical attributes. A breakpoint is inserted here so that you can have a look at the ExampleSet. You can see that the ExampleSet has three attributes and each attribute has three possible values. The ID3 operator is applied on this ExampleSet with default values of all parameters. The resultant Decision Tree model is delivered to the result port of the process and it can be seen in the Results Workspace.