One-vs-One Multiclass

This article describes how to use the One-vs-One Multiclass component in Azure Machine Learning designer. The goal is to create a classification model that can predict multiple classes, by using the one-versus-one approach.

This component is useful for creating models that predict three or more possible outcomes, when the outcome depends on continuous or categorical predictor variables. This method also lets you use binary classification methods for issues that require multiple output classes.

More about one-versus-one models

Some classification algorithms permit the use of more than two classes by design. Others restrict the possible outcomes to one of two values (a binary, or two-class model). But even binary classification algorithms can be adapted for multi-class classification tasks through a variety of strategies.

This component implements the one-versus-one method, in which a binary model is created per class pair. At prediction time, the class which received the most votes is selected. Since it requires to fit n_classes * (n_classes - 1) / 2 classifiers, this method is usually slower than one-versus-all, due to its O(n_classes^2) complexity. However, this method may be advantageous for algorithms such as kernel algorithms which don’t scale well with n_samples. This is because each individual learning problem only involves a small subset of the data whereas, with one-versus-all, the complete dataset is used n_classes times.

In essence, the component creates an ensemble of individual models and then merges the results, to create a single model that predicts all classes. Any binary classifier can be used as the basis for a one-versus-one model.

For example, let’s say you configure a Two-Class Support Vector Machine model and provide that as input to the One-vs-One Multiclass component. The component would create two-class support vector machine models for all members of the output class. It would then apply the one-versus-one method to combine the results for all classes.

The component uses OneVsOneClassifier of sklearn, and you can learn more details here.

How to configure the One-vs-One Multiclass classifier

This component creates an ensemble of binary classification models to analyze multiple classes. To use this component, you need to configure and train a binary classification model first.

You connect the binary model to the One-vs-One Multiclass component. You then train the ensemble of models by using Train Model with a labeled training dataset.

When you combine the models, One-vs-One Multiclass creates multiple binary classification models, optimizes the algorithm for each class, and then merges the models. The component does these tasks even though the training dataset might have multiple class values.

1. Add the One-vs-One Multiclass component to your pipeline in the designer. You can find this component under Machine Learning - Initialize, in the Classification category.

   The One-vs-One Multiclass classifier has no configurable parameters of its own. Any customizations must be done in the binary classification model that's provided as input.

2. Add a binary classification model to the pipeline, and configure that model. For example, you might use Two-Class Support Vector Machine or Two-Class Boosted Decision Tree.

3. Add the Train Model component to your pipeline. Connect the untrained classifier that is the output of One-vs-One Multiclass.

4. On the other input of Train Model, connect a labeled training dataset that has multiple class values.

5. Submit the pipeline.

Results

After training is complete, you can use the model to make multiclass predictions.

Alternatively, you can pass the untrained classifier to Cross-Validate Model for cross-validation against a labeled validation dataset.

Next steps

See the set of components available to Azure Machine Learning.