Dynamic Classifier Selection Ensembles in Python



Dynamic classifier selection is a type of ensemble learning algorithm for classification predictive modeling.

The technique involves fitting multiple machine learning models on the training dataset, then selecting the model that is expected to perform best when making a prediction, based on the specific details of the example to be predicted.

This can be achieved using a k-nearest neighbor model to locate examples in the training dataset that are closest to the new example to be predicted, evaluating all models in the pool on this neighborhood and using the model that performs the best on the neighborhood to make a prediction for the new example.

As such, the dynamic classifier selection can often perform better than any single model in the pool and provides an alternative to averaging the predictions from multiple models, as is the case in other ensemble algorithms.

In this tutorial, you will discover how to develop dynamic classifier selection ensembles in Python.

After completing this tutorial, you will know:

  • Dynamic classifier selection algorithms choose one from among many models to make a prediction for each new example.
  • How to develop and evaluate dynamic classifier selection models for classification tasks using the scikit-learn API.
  • How to explore the effect of dynamic classifier selection model hyperparameters on classification accuracy.

Let’s get started.

How to Develop Dynamic Classifier Selection in Python

How to Develop Dynamic Classifier Selection in Python
Photo by Jean and Fred, some rights reserved.

Tutorial Overview

This tutorial is divided into three parts; they are:

  1. Dynamic Classifier Selection
  2. Dynamic Classifier Selection With Scikit-Learn
    1. DCS With Overall Local Accuracy (OLA)
    2. DCS With Local Class Accuracy (LCA)
  3. Hyperparameter Tuning for DCS
    1. Explore k in k-Nearest Neighbor
    2. Explore Algorithms for Classifier Pool

Dynamic Classifier Selection

Multiple Classifier Systems refers to a field of machine learning algorithms that use multiple models to address classification predictive modeling problems.

This includes familiar techniques such as one-vs-rest, one-vs-all, and output error-correcting codes techniques. It also includes more general techniques that select a model to use dynamically for each new example that requires a prediction.

Several approaches are currently used to construct an MCS […] One of the most promising MCS approaches is Dynamic Selection (DS), in which the base classifiers are selected on the fly, according to each new sample to be classified.

Dynamic Classifier Selection: Recent Advances And Perspectives, 2018.

For more on these types of multiple classifier systems, see the tutorial:

These methods are generally known by the name: Dynamic Classifier Selection, or DCS for short.

  • Dynamic Classifier Selection: Algorithms that choose one from among many trained models to make a prediction based on the specific details of the input.

Given that multiple models are used in DCS, it is considered a type of ensemble learning technique.

Dynamic Classifier Selection algorithms generally involve partitioning the input feature space in some way and assigning specific models to be responsible for making predictions for each partition. There are a variety of different DCS algorithms and research efforts are mainly focused on how to evaluate and assign classifiers to specific regions of the input space.

After training multiple individual learners, DCS dynamically selects one learner for each test instance. […] DCS makes predictions by using one individual learner.

— Page 93, Ensemble Methods: Foundations and Algorithms, 2012.

An early and popular approach involves first fitting a small, diverse set of classification models on the training dataset. When a prediction is required, first a k-nearest neighbor (kNN) algorithm is used to find the k most similar examples from the training dataset that match the example. Each previously fit classifier in the model is then evaluated on the neighbor of k training examples and the classifier that performs the best is selected to make a prediction for the new example.

This approach is referred to as “Dynamic Classifier Selection Local Accuracy” or DCS-LA for short and was described by Kevin Woods, et al. in their 1997 paper titled “Combination Of Multiple Classifiers Using Local Accuracy Estimates.”

The basic idea is to estimate each classifier’s accuracy in local region of feature space surrounding an unknown test sample, and then use the decision of the most locally accurate classifier.

Combination of multiple classifiers using local accuracy estimates, 1997.

The authors describe two approaches for selecting a single classifier model to make a prediction for a given input example, they are:

  • Local Accuracy, often referred to as LA or Overall Local Accuracy (OLA).
  • Class Accuracy, often referred to as CA or Local Class Accuracy (LCA).

Local Accuracy (OLA) involves evaluating the classification accuracy of each model on the neighborhood of k training examples. The model that performs the best in this neighborhood is then selected to make a prediction for the new example.

The OLA of each classifier is computed as the percentage of the correct recognition of the samples in the local region.

Dynamic Selection Of Classifiers—a Comprehensive Review, 2014.

Class Accuracy (LCA) involves using each model to make a prediction for the new example and noting the class that was predicted. Then, the accuracy of each model on the neighbor of k training examples is evaluated and the model that has the best skill for the class that it predicted on the new example is selected and its prediction returned.

The LCA is estimated for each base classifier as the percentage of correct classifications within the local region, but considering only those examples where the classifier has given the same class as the one it gives for the unknown pattern.

Dynamic Selection Of Classifiers—a Comprehensive Review, 2014.

In both cases, if all fit models make the same prediction for a new input example, then the prediction is returned directly.

Now that we are familiar with DCS and the DCS-LA algorithm, let’s look at how we can use it on our own classification predictive modeling projects.

Dynamic Classifier Selection With Scikit-Learn

The Dynamic Ensemble Selection Library or DESlib for short is an open source Python library that provides an implementation of many different dynamic classifier selection algorithms.

DESlib is an easy-to-use ensemble learning library focused on the implementation of the state-of-the-art techniques for dynamic classifier and ensemble selection.

First, we can install the DESlib library using the pip package manager.

Once installed, we can confirm that the library was installed correctly and is ready to be used by loading the library and printing the installed version.

Running the script will print your version of the DESlib library you have installed.

Your version should be the same or higher. If not, you must upgrade your version of the DESlib library.

The DESlib provides an implementation of the DCS-LA algorithm with each classifier selection technique via the OLA and LCA classes respectively.

Each class can be used as a scikit-learn model directly, allowing the full suite of scikit-learn data preparation, modeling pipelines, and model evaluation techniques to be used directly.

Both classes use a k-nearest neighbor algorithm to select the neighbor with a default value of k=7.

A bootstrap aggregation (bagging) ensemble of decision trees is used as the pool of classifier models considered for each classification that is made by default, although this can be changed by setting “pool_classifiers” to a list of models.

We can use the make_classification() function to create a synthetic binary classification problem with 10,000 examples and 20 input features.

Running the example creates the dataset and summarizes the shape of the input and output components.

Now that we are familiar with the DESlib API, let’s look at how to use each DCS-LA algorithm.

DCS With Overall Local Accuracy (OLA)

We can evaluate a DCS-LA model using overall local accuracy on the synthetic dataset.

In this case, we will use default model hyperparameters, including bagged decision trees as the pool of classifier models and a k=7 for the selection of the local neighborhood when making a prediction.

We will evaluate the model using repeated stratified k-fold cross-validation with three repeats and 10 folds. We will report the mean and standard deviation of the accuracy of the model across all repeats and folds.

The complete example is listed below.

Running the example reports the mean and standard deviation accuracy of the model.

Note: Your results may vary given the stochastic nature of the algorithm or evaluation procedure, or differences in numerical precision. Consider running the example a few times and compare the average outcome.

In this case, we can see the DCS-LA with OLA and default hyperparameters achieves a classification accuracy of about 88.3 percent.

We can also use the DCS-LA model with OLA as a final model and make predictions for classification.

First, the model is fit on all available data, then the predict() function can be called to make predictions on new data.

The example below demonstrates this on our binary classification dataset.

Running the example fits the DCS-LA with OLA model on the entire dataset and is then used to make a prediction on a new row of data, as we might when using the model in an application.

Now that we are familiar with using DCS-LA with OLA, let’s look at the LCA method.

DCS With Local Class Accuracy (LCA)

We can evaluate a DCS-LA model using local class accuracy on the synthetic dataset.

In this case, we will use default model hyperparameters, including bagged decision trees as the pool of classifier models and a k=7 for the selection of the local neighborhood when making a prediction.

We will evaluate the model using repeated stratified k-fold cross-validation with three repeats and 10 folds. We will report the mean and standard deviation of the accuracy of the model across all repeats and folds.

The complete example is listed below.

Running the example reports the mean and standard deviation accuracy of the model.

Note: Your results may vary given the stochastic nature of the algorithm or evaluation procedure, or differences in numerical precision. Consider running the example a few times and compare the average outcome.

In this case, we can see the DCS-LA with LCA and default hyperparameters achieves a classification accuracy of about 92.2 percent.

We can also use the DCS-LA model with LCA as a final model and make predictions for classification.

First, the model is fit on all available data, then the predict() function can be called to make predictions on new data.

The example below demonstrates this on our binary classification dataset.

Running the example fits the DCS-LA with LCA model on the entire dataset and is then used to make a prediction on a new row of data, as we might when using the model in an application.

Now that we are familiar with using the scikit-learn API to evaluate and use DCS-LA models, let’s look at configuring the model.

Hyperparameter Tuning for DCS

In this section, we will take a closer look at some of the hyperparameters you should consider tuning for the DCS-LA model and their effect on model performance.

There are many hyperparameters we can look at for DCS-LA, although in this case, we will look at the value of k in the k-nearest neighbor model used in the local evaluation of the models, and how to use a custom pool of classifiers.

We will use the DCS-LA with OLA as the basis for these experiments, although the choice of the specific method is arbitrary.

Explore k in k-Nearest Neighbor

The configuration of the k-nearest neighbor algorithm is critical to the DCS-LA model as it defines the scope of the neighborhood in which each classifier is considered for selection.

The k value controls the size of the neighborhood and it is important to set it to a value that is appropriate for your dataset, specifically the density of samples in the feature space. A value too small will mean that relevant examples in the training set might be excluded from the neighborhood, whereas values too large may mean that the signal is being washed out by too many examples.

The example below explores the classification accuracy of the DCS-LA with OLA with k values from 2 to 21.

Running the example first reports the mean accuracy for each configured neighborhood size.

Note: Your results may vary given the stochastic nature of the algorithm or evaluation procedure, or differences in numerical precision. Consider running the example a few times and compare the average outcome.

In this case, we can see that accuracy increases with the neighborhood size, perhaps to k=13 or k=14, where it appears to level off.

A box and whisker plot is created for the distribution of accuracy scores for each configured neighborhood size.

We can see the general trend of increasing model performance and k value before reaching a plateau.

Box and Whisker Plots of Accuracy Distributions for k Values in DCS-LA With OLA

Box and Whisker Plots of Accuracy Distributions for k Values in DCS-LA With OLA

Explore Algorithms for Classifier Pool

The choice of algorithms used in the pool for the DCS-LA is another important hyperparameter.

By default, bagged decision trees are used, as it has proven to be an effective approach on a range of classification tasks. Nevertheless, a custom pool of classifiers can be considered.

This requires first defining a list of classifier models to use and fitting each on the training dataset. Unfortunately, this means that the automatic k-fold cross-validation model evaluation methods in scikit-learn cannot be used in this case. Instead, we will use a train-test split so that we can fit the classifier pool manually on the training dataset.

The list of fit classifiers can then be specified to the OLA (or LCA) class via the “pool_classifiers” argument. In this case, we will use a pool that includes logistic regression, a decision tree, and a naive Bayes classifier.

The complete example of evaluating DCS-LA with OLA and a custom set of classifiers on the synthetic dataset is listed below.

Running the example first reports the mean accuracy for the model with the custom pool of classifiers.

Note: Your results may vary given the stochastic nature of the algorithm or evaluation procedure, or differences in numerical precision. Consider running the example a few times and compare the average outcome.

In this case, we can see that the model achieved an accuracy of about 91.2 percent.

In order to adopt the DCS model, it must perform better than any contributing model. Otherwise, we would simply use the contributing model that performs better instead.

We can check this by evaluating the performance of each contributing classifier on the test set.

The updated example of DCS-LA with a custom pool of classifiers that are also evaluated independently is listed below.

Running the example first reports the mean accuracy for the model with the custom pool of classifiers and the accuracy of each contributing model.

Note: Your results may vary given the stochastic nature of the algorithm or evaluation procedure, or differences in numerical precision. Consider running the example a few times and compare the average outcome.

In this case, we can see that again, the DCS-LA achieves an accuracy of about 91.3 percent, which is better than any contributing model.

Further Reading

This section provides more resources on the topic if you are looking to go deeper.

Related Tutorials

Papers

Books

APIs

Summary

In this tutorial, you discovered how to develop dynamic classifier selection ensembles in Python.

Specifically, you learned:

  • Dynamic classifier selection algorithms choose one from among many models to make a prediction for each new example.
  • How to develop and evaluate dynamic classifier selection models for classification tasks using the scikit-learn API.
  • How to explore the effect of dynamic classifier selection model hyperparameters on classification accuracy.

Do you have any questions?
Ask your questions in the comments below and I will do my best to answer.



Author: Shantun Parmar

Comments are closed.