AdaBoost (RapidMiner Studio Core)
SynopsisThis operator is an implementation of the AdaBoost algorithm and it can be used with all learners available in RapidMiner. AdaBoost is a meta-algorithm which can be used in conjunction with many other learning algorithms to improve their performance.
The AdaBoost operator is a nested operator i.e. it has a subprocess. The subprocess must have a learner i.e. an operator that expects an ExampleSet and generates a model. This operator tries to build a better model using the learner provided in its subprocess. You need to have a basic understanding of subprocesses in order to apply this operator. Please study the documentation of the Subprocess operator for basic understanding of subprocesses.
AdaBoost, short for Adaptive Boosting, is a meta-algorithm, and can be used in conjunction with many other learning algorithms to improve their performance. AdaBoost is adaptive in the sense that subsequent classifiers built are tweaked in favor of those instances misclassified by previous classifiers. AdaBoost is sensitive to noisy data and outliers. In some problems, however, it can be less susceptible to the overfitting problem than most learning algorithms. The classifiers it uses can be weak (i.e., display a substantial error rate), but as long as their performance is not random (resulting in an error rate of 0.5 for binary classification), they will improve the final model.
AdaBoost generates and calls a new weak classifier in each of a series of rounds t = 1,…,T . For each call, a distribution of weights D(t) is updated that indicates the importance of examples in the data set for the classification. On each round, the weights of each incorrectly classified example are increased, and the weights of each correctly classified example are decreased, so the new classifier focuses on the examples which have so far eluded correct classification.
Ensemble Theory Boosting is an ensemble method, therefore an overview of the Ensemble Theory has been discussed here. Ensemble methods use multiple models to obtain better predictive performance than could be obtained from any of the constituent models. In other words, an ensemble is a technique for combining many weak learners in an attempt to produce a strong learner. Evaluating the prediction of an ensemble typically requires more computation than evaluating the prediction of a single model, so ensembles may be thought of as a way to compensate for poor learning algorithms by performing a lot of extra computation.
An ensemble is itself a supervised learning algorithm, because it can be trained and then used to make predictions. The trained ensemble, therefore, represents a single hypothesis. This hypothesis, however, is not necessarily contained within the hypothesis space of the models from which it is built. Thus, ensembles can be shown to have more flexibility in the functions they can represent. This flexibility can, in theory, enable them to over-fit the training data more than a single model would, but in practice, some ensemble techniques (especially bagging) tend to reduce problems related to over-fitting of the training data.
Empirically, ensembles tend to yield better results when there is a significant diversity among the models. Many ensemble methods, therefore, seek to promote diversity among the models they combine. Although perhaps non-intuitive, more random algorithms (like random decision trees) can be used to produce a stronger ensemble than very deliberate algorithms (like entropy-reducing decision trees). Using a variety of strong learning algorithms, however, has been shown to be more effective than using techniques that attempt to dumb-down the models in order to promote diversity.
- 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.
- model (Ada Boost Model)
The meta model is delivered from this output port which can now be applied on unseen data sets for 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.
- iterationsThis parameter specifies the maximum number of iterations of the AdaBoost algorithm. Range: integer
Using the AdaBoost operator for generating a better Decision Tree
The 'Sonar' data set is loaded using the Retrieve operator. The Split Validation operator is applied on it for training and testing a classification model. The AdaBoost operator is applied in the training subprocess of the Split Validation operator. The Decision Tree operator is applied in the subprocess of the AdaBoost operator. The iterations parameter of the AdaBoost operator is set to 10, thus there will be at maximum 10 iterations of its subprocess. The Apply Model operator is used in the testing subprocess for applying the model generated by the AdaBoost operator. The resultant labeled ExampleSet is used by the Performance (Classification) operator for measuring the performance of the model. The classification model and its performance vector is connected to the output and it can be seen in the Results Workspace. You can see that the AdaBoost operator produced a new model in each iteration and there are different weights for each model. The accuracy of this model turns out to be around 69%. If the same process is repeated without AdaBoost operator i.e. only the Decision Tree operator is used in training subprocess. The accuracy of that model turns out to be around 66%. Thus AdaBoost generated a combination of models that performed better than the original model.