The effect of oversampling and undersampling on classifying imbalanced text datasets

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Liu, Alexander Yun-chung

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Many machine learning classification algorithms assume that the target classes share similar prior probabilities and misclassification costs. However, this is often not the case in the real world. The problem of classification when one class has a much lower prior probability in the training set is called the imbalanced dataset problem. One popular approach to solving the imbalanced dataset problem is to resample the training set. However, few studies in the past have considered resampling algorithms on data sets with high dimensionality. In this thesis, we examine the imbalanced dataset problem in the realm of text classification. Text has the added problems of both sparsity and high dimensionality. We first describe the resampling techniques we use in this thesis, including several resampling techniques we are introducing. After resampling, we classify the data using multinomial naïve Bayes, k nearest neighbor, and SVMs. Finally, we compare the results of our experiments and find that, while the best resampling technique to use is often dataset dependent, certain resampling techniques tend to perform consistently when coupled with certain classifiers


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