Statistical Learning Approaches to Some Classification Problems

Classification is essential in statistical learning. This thesis deals with three topics in classification: multi-label classification, nonparametric multi-class classification and a special type of text categorization called occupation coding. For each topic, novel approaches are proposed with the goal of high predictive performance. This is empirically demonstrated for each method. In multi-label classification, observations may be associated with multiple classes or labels simultaneously. Generally, correlations exist between labels and taking into account the label correlations is important during the classification process. This thesis proposes an approach based on the nearest neighbor principle that considers neighbors both in the feature (x) and the label (y) space. The proposed method chooses the labelset of a training observation that minimizes a weighted function of the distances in feature and label space. By selecting an entire labelset as the prediction, the method implicitly considers label correlations. In multi-class classification, the well-known k-nearest neighbors method is especially desirable when the response surface exhibits highly local behavior. A novel approach is presented that makes a prediction based on the k-th nearest neighbor from each class. The method not only provides estimates for class posterior probabilities but also converges to the Bayes classifier as the size of the training data increases. Further, the method is extended using the idea of an ensemble. Occupation coding is an important multi-class text categorization problem. Since fully automated classification is challenging, researchers focus more on partially automated coding. Three approaches based on underlying statistical learning methods are proposed to improve the classification accuracy of the underlying statistical learning methods.