active learning for class imbalance problem. problem to be addressed motivation class imbalance...
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Active Learning for Class Imbalance Problem
Problem to be addressed Motivation
class imbalance problem referring to the situation that at least one of class having significantly
less number of training examples or examples in training data belonging to one class heavily outnumber
the examples in the other class
Currently, most of the machine learning algorithms assume the training data to be balanced, support vector machine, logistic regression, naïve bayesian classifier etc,.
During the last few decades, some effective methods have been proposed to attack this problem, like up-sampling, down-sampling and asymmetric bagging, etc,.
Problem to be addressed
Detailed problem Traditional machine learning algorithms are often
biased toward the majority class
Since the goal of the classifiers is to reduce the training error, not taking the data distribution into consideration
Consequently, examples from the majority class are well-classified while the examples from minority class tend to be misclassified
Several Common Approaches
From the data perspective Over-sampling Under-sampling Asymmetric Bagging
From the learning algorithm perspective Adjusting the cost function Tuning the related parameters
Background Knowledge Active Learning
Similar to semi-supervised learning method, the key idea is to use both the labeled and unlabeled data for classifier training.
Active learning is composed of four components A small set of labeled training data, a large pool of unlabeled data, a
based learning algorithm and an active learner (selection strategy)
Active learning is not a machine learning algorithm, It can be seen as a enhancing wrapper method
The difference between semi-supervised learning and active learning
Background Knowledge
Active Learning Goals of active learning
Maximizing the learning performance while minimizing the required labeled training examples
Achieving better performance using the same amount of labeled training data
Needing less training samples to obtain the same learning performance
Background Knowledge
Background Knowledge
An Example
SVM-based Active Learning A small set of labeled training examples A large pool of unlabeled data Base learning algorithm SVM Active Learner (selection strategy)
Instances closest to the current separating hyperplane are selected and asks for human labeling
Problems
SVM-based Active Learning In classical active learning methods, the most informative samples
are selected from the entire unlabeled pool
In other words, each iteration of active learning involves the computation of distance of each sample to the decision boundary
For large-scale data set, it is time-consuming and computationally inefficient
Paper Contribution
Proposed method Instead of querying the whole unlabeled
pool , a subset is first selected
Select the closed sample from using the criterion that is among the top closest instances with probability
Paper Contribution
Proposed Method The probability that at least one of the L
instances is among the closest is We have
Paper Contribution
Proposed Method For example
The active learner will pick one instance, with 95% probability, that is among the top 5% closed instances to the separating hyperplane, by randomly sampling only instances regardless of the training set size
Experiments
Experiments Evaluation Metric
g-means
where sensitivity and specifity are the accuracies of the positive and negative instances respectively
Experiments
Experiments
Experiments
Experiments
Conclusions This paper propose a method to address the class
imbalance problem using active learning technique
Experimental results show that this approach can achieve a significant decrease in the training time, while maintaining the same or even higher g-means value by using less number of training examples
Active selection of informative examples from a randomly selected subset avoid searching the whole unlabeled pool