Datasets produced in modern research, such as biomedical science, pose a number of challenges for machine learning techniques used in binary classification due to high dimensionality. Feature selection is one of the most important statistical techniques used for dimensionality reduction of the datasets. Therefore, techniques are needed to find an optimal number of features to obtain more desirable learning performance. In the machine learning context, gene selection is treated as a feature selection problem, the objective of which is to find a small subset of the most discriminative features for the target class. In this paper, a gene selection method is proposed that identifies the most discriminative genes in two stages. Genes that unambiguously assign the maximum number of samples to their respective classes using a greedy approach are selected in the first stage. The remaining genes are divided into a certain number of clusters. From each cluster, the most informative genes are selected via the lasso method and combined with genes selected in the first stage. The performance of the proposed method is assessed through comparison with other stateof-the-art feature selection methods using gene expression datasets. This is done by applying two classifiers i.e., random forest and support vector machine, on datasets with selected genes and training samples and calculating their classification accuracy, sensitivity, and Brier score on samples in the testing part. Boxplots based on the results and correlation matrices of the selected genes are thenceforth constructed. The results show that the proposed method outperforms the other methods. INDEX TERMS Clustering, classification, feature selection, high dimensional data, microarray gene expression data.

Ensemble methods based on k-NN models minimise the effect of outliers in a training dataset by searching groups of the k closest data points to estimate the response of an unseen observation. However, traditional k-NN based ensemble methods use the arithmetic mean of the training points' responses for estimation which has several weaknesses. Traditional k-NN based models are also adversely affected by the presence of non-informative features in the data. This paper suggests a novel ensemble procedure consisting of a class of base k-NN models each constructed on a bootstrap sample drawn from the training dataset with a random subset of features. In the k nearest neighbours determined by each k-NN model, stepwise regression is fitted to predict the test point. The final estimate of the target observation is then obtained by averaging the estimates from all the models in the ensemble. The proposed method is compared with some other state-of-the-art procedures on 16 benchmark datasets in terms of coefficient of determination (R 2 ), Pearson's product-moment correlation coefficient (r), mean square predicted error (M SP E), root mean squared error (RM SE) and mean absolute error (M AE) as performance metrics. Furthermore, boxplots of the results are also constructed. The suggested ensemble procedure has outperformed the other procedures on almost all the datasets. The efficacy of the method has also been verified by assessing the proposed method in comparison with the other methods by adding non-informative features to the datasets considered. The results reveal that the proposed method is more robust to the issue of non-informative features in the data as compared to the rest of the methods.

In this paper, a novel feature selection method called Robust Proportional Overlapping Score (RPOS), for microarray gene expression datasets has been proposed, by utilizing the robust measure of dispersion, i.e., Median Absolute Deviation (MAD). This method robustly identifies the most discriminative genes by considering the overlapping scores of the gene expression values for binary class problems. Genes with a high degree of overlap between classes are discarded and the ones that discriminate between the classes are selected. The results of the proposed method are compared with five state-of-the-art gene selection methods based on classification error, Brier score, and sensitivity, by considering eleven gene expression datasets. Classification of observations for different sets of selected genes by the proposed method is carried out by three different classifiers, i.e., random forest, k-nearest neighbors (k-NN), and support vector machine (SVM). Box-plots and stability scores of the results are also shown in this paper. The results reveal that in most of the cases the proposed method outperforms the other methods.

kNN based ensemble methods minimise the effect of outliers by identifying a set of data points in the given feature space that are nearest to an unseen observation in order to predict its response by using majority voting. The ordinary ensembles based on kNN find out the k nearest observations in a region (bounded by a sphere) based on a predefined value of k. This scenario, however, might not work in situations when the test observation follows the pattern of the closest data points with the same class that lie on a certain path not contained in the given sphere. This paper proposes a k nearest neighbour ensemble where the neighbours are determined in k steps. Starting from the first nearest observation of the test point, the algorithm identifies a single observation that is closest to the observation at the previous step. At each base learner in the ensemble, this search is extended to k steps on a random bootstrap sample with a random subset of features selected from the feature space. The final predicted class of the test point is determined by using a majority vote in the predicted classes given by all base models. This new ensemble method is applied on 17 benchmark datasets and compared with other classical methods, including kNN based models, in terms of classification accuracy, kappa and Brier score as performance metrics. Boxplots are also utilised to illustrate the difference in the results given by the proposed and other state-of-the-art methods. The proposed method outperformed the rest of the classical methods in the majority of cases. The paper gives a detailed simulation study for further assessment.

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