example2.rst

Example 2 : Understanding the hyper-parameter optimization

Intuitive explanation on hyper-parameters

Hyper-parameters are parameters of a classifier (monoview or multiview) that are task-dependant and have a huge part in the performance of the algorithm for a given task.

The simplest example is the decision tree. One of it's hyper-parameter is the depth of the tree. The deeper the tree is, the most it will fit on the learning data. However a tree too deep will most likely overfit and won't have any value on unseen testing data.

This platform proposes a randomized search for optimizing hyperparamter on the given task. In this example, we first will analyze how it works and then how to use it.

Understanding train/test split

In order to provide robust results, this platform splits the dataset in a training set, tha will be used by the classifier to optimize their hyper-parameter and learn a relevant model, and a testing set that will take no part in the learning process and serve as unseen data to estimate each model's generalization capacity.

This split is controlled by the config file's argument split:. It uses a float to pass the ratio between the size of the testing set and the training set : \text{split} = \frac{\text{test size}}{\text{train size}}. In order to be as fare as possible, this split is made by keeping the ratio btween each class in the training set and in the testing set.

So if a dataset has 100 examples with 60% of them in class A, and 40% of them in class B, using split: 0.2 will generate a training set with 48 examples of class A and 32 examples of class B and a testing set with 12 examples of class A and 8 examples of class B.

Ths process uses sklearn's StratifiedShuffleSplit to split the dataset at random while being reproductilbe thanks to the random state.

Understanding hyper-parameter optimization

As hyper-paramters are task dependant, there are two ways in the platform to set their value :

• If you know the value (or a set of values), specify them at the end of the config file for each algorithm you want to test, and use hps_type: None in the classifiaction section of the config file. This will set the Hyper Parameter Search to None, and bypass the optimization process to run the algorithm on the specified values.
• If you don't know the value, the platform proposes a random search for hyper-parameter optimization.

Random search

The random search is one of the most efficient while fairest method to optimize hyper-parameter. Thus, for each algorithm in the platform, each of its hyper-paramter is provided with distribution of possible values, (for example, the decision tree's max depth parameter is provided with a uniform distribution between 1 and 300). The random search method will randomly select hyper-parameters within this distribution and evaluate the performance of the classifier with this configuration. It will repeat that process with different randomly selected sets of hyper-parameter and keep the best configuration performance-wise.

In the config file, to enable random search, set the hps_type: line to hps_type: "randomized_search" and to control the number of draws, use the hps_iter: line.

K-folds cross-validation

During the process of optimizing the hyper-parameter, the random serach has to estimate the perofmance of the classifier.

In order to do so, the platform uses k-folds cross-validation. This method consists in splitting the training set in k equal sub-sets, training the classifier (with the randomly chose hyper-parameters) on k-1 subsets an testing it on the last one, evaluating it's predictive performance.

This learning-and-testing process is repeated k times and the estimated performance is the mean of the performance on each testing set.

In the platform, the training set (the 48 examples of class A and 32 examples of class B from last example) will be divided in k folds for the cross-validation process and the testing set (the 12 examples of class A and 8 examples of class B for last examples) will in no way be involved in the training process of the classifier.

The cross-validation process can be controled with the nb_folds: line of the configuration file in which the number of folds is specified.

Metric choice

This hyper-parameter optimization can be strongly metric-dependant. For example, for an unbalanced dataset, evaluating the accuracy is not relevant and will not provide a good estimation of the performance of the classifier. In the platform, it is possible to specify the metric that will be used for the hyper-parameter optimization process thanks to the metric_princ: line in the configuration file.

Hands-on experience

In order to understand the process and it's usefulness, let's run some configurations and analyze the results.

This example will focus only on some lines of the configuration file :

• split:, controlling the ration of size between the testing set and the training set,
• hps_type:, controlling the type of hyper-parameter search,
• hps_iter:, controlling the number of random draws during the hyper-parameter search,
• nb_folds:, controlling the number of folds in the cross-validation process.

Example 2.1 : No hyper-parameter optimization, impact of split size

For this example, we only used a subset of the available classifiers, to reduce the computation time and the complexity of the results.

Each classifier will first be learned on the default hyper-parameters (as in Example 1)

The monoview classifiers that will be used are adaboost and decision_tree, and the multivew classifier is a late fusion majority vote.

In the config file, the default values for adaboost's hyper-parameters are :

adaboost:
    n_estimators: [50]
    base_estimator: ["DecisionTreeClassifier"]

(see adaboost's sklearn's page for more information)

For decision_tree :

decision_tree:
    max_depth: [10]
    criterion: ["gini"]
    splitter: ["best"]

(sklearn's decision tree)

And for the late fusion majority vote :

majority_voting_fusion:
    classifier_names: [["decision_tree", "decision_tree", "decision_tree", ]]
    classifier_configs:
        decision_tree:
            max_depth: [1]
            criterion: ["gini"]
            splitter: ["best"]

(It will build a vote with one decision tree on each view, with the specified configuration for the decision trees)

To run this example,