The purpose of sentiment classification is to determine whether a particular document has a positive or negative nuance. Sentiment classification is extensively used in many business domains to improve products or services by understanding the opinions of customers regarding these products. Deep learning achieves state-of-the-art results in various challenging domains. With the success of deep learning, many studies have proposed deep-learning-based sentiment classification models and achieved better performances compared with conventional machine learning models. However, one practical issue occurring in deep-learning-based sentiment classification is that the best model structure depends on the characteristics of the dataset on which the deep learning model is trained; moreover, it is manually determined based on the domain knowledge of an expert or selected from a grid search of possible candidates. Herein, we present a comparative study of different deep-learning-based sentiment classification model structures to derive meaningful implications for building sentiment classification models. Specifically, eight deeplearning models, three based on convolutional neural networks and five based on recurrent neural networks, with two types of input structures, i.e., word level and character level, are compared for 13 review datasets, and the classification performances are discussed under different perspectives. INDEX TERMS Sentiment classification, deep learning, convolutional neural network, recurrent neural network, word embedding, character embedding.

Previous methods for system intrusion detection have mainly consisted of those based on pattern matching that employs prior knowledge extracted from experts' domain knowledge. However, pattern matching-based methods have a major drawback that it can be bypassed through various modified techniques. These advanced persistent threats cause limitation to the pattern matching-based detecting mechanism, because they are not only more sophisticated than usual threats but also specialized in the targeted attacking object. The defense mechanism should have to comprehend unusual phenomenons or behaviors to successfully handles the advanced threats. To achieve this, various security techniques based on machine learning have been developed recently. Among these, anomaly detection algorithms, which are trained in unsupervised fashion, are capable of reducing efforts of security experts and securing labeled dataset through post analysis. It is further possible to distinguish abnormal behaviors more precisely by training classification models if sufficient amounts of labeled dataset is obtained through post analysis of anomaly detection results. In this study, we proposed an end-to-end abnormal behavior detection method based on sequential information preserving log embedding algorithms and machine learning-based anomaly detection algorithms. Contrary to other machine learning based system anomaly detection models, which borrow domain experts' knowledge to extract significant features from the log data, raw log data are transformed into a fixed size of continuous vector regardless of their length, and these vectors are used to train the anomaly detection models. Experimental results based on a real system call trace dataset, our proposed log embedding method with unsupervised anomaly detection model yielded a favorable performance, at most 0.8708 in terms of AUROC, and it can be further improved up to 0.9745 with supervised classification algorithms if sufficient labeled attack log data become available.