Abstract: Ubuntu dialogue corpus is the largest public available dialogue corpus to make it feasible to build end-to-end deep neural network models directly from the conversation data. One challenge of Ubuntu dialogue corpus is the large number of out-of-vocabulary words. In this paper we proposed a method which combines the general pre-trained word embedding vectors with those generated on the task-specific training set to address this issue. We integrated character embedding into Chen et al's Enhanced LSTM method (ESIM) and used it to evaluate the effectiveness of our proposed method. For the task of next utterance selection, the proposed method has demonstrated a significant performance improvement against original ESIM and the new model has achieved state-of-the-art results on both Ubuntu dialogue corpus and Douban conversation corpus. In addition, we investigated the performance impact of end-of-utterance and end-of-turn token tags.

Introduction: The ability for a machine to converse with human in a natural and coherent manner is one of challenging goals in AI and natural language understanding. One problem in chat-oriented human-machine dialog system is to reply a message within conversation contexts. Existing methods can be divided into two categories: retrieval-based methods BIBREF0 , BIBREF1 , BIBREF2 and generation based methods BIBREF3 . The former is to rank a list of candidates and select a good response. For the latter, encoder-decoder framework BIBREF3 or statistical translation method BIBREF4 are usually used to generate a response. It is not easy to main the fluency of the generated texts. Ubuntu dialogue corpus BIBREF5 is the public largest unstructured multi-turns dialogue corpus which consists of about one-million two-person conversations. The size of the corpus makes it attractive for the exploration of deep neural network modeling in the context of dialogue systems. Most deep neural networks use word embedding as the first layer. They either use fixed pre-trained word embedding vectors generated on a large text corpus or learn word embedding for the specific task. The former is lack of flexibility of domain adaptation. The latter requires a very large training corpus and significantly increases model training time. Word out-of-vocabulary issue occurs for both cases. Ubuntu dialogue corpus also contains many technical words (e.g. “ctrl+alt+f1", “/dev/sdb1"). The ubuntu corpus (V2) contains 823057 unique tokens whereas only 22% tokens occur in the pre-built GloVe word vectors. Although character-level representation which models sub-word morphologies can alleviate this problem to some extent BIBREF6 , BIBREF7 , BIBREF8 , character-level representation still have limitations: learn only morphological and orthographic similarity, other than semantic similarity (e.g. `car' and `bmw') and it cannot be applied to Asian languages (e.g. Chinese characters). In this paper, we generate word embedding vectors on the training corpus based on word2vec BIBREF9 . Then we propose an algorithm to combine the generated one with the pre-trained word embedding vectors on a large general text corpus based on vector concatenation. The new word representation maintains information learned from both general text corpus and task-domain. The nice property of the algorithm is simplicity and little extra computational cost will be added. It can address word out-of-vocabulary issue effectively. This method can be applied to most NLP deep neural network models and is language-independent. We integrated our methods with ESIM(baseline model) BIBREF10 . The experimental results have shown that the proposed method has significantly improved the performance of original ESIM model and obtained state-of-the-art results on both Ubuntu Dialogue Corpus and Douban Conversation Corpus BIBREF11 . On Ubuntu Dialogue Corpus (V2), the improvement to the previous best baseline model (single) on INLINEFORM0 is 3.8% and our ensemble model on INLINEFORM1 is 75.9%. On Douban Conversation Corpus, the improvement to the previous best model (single) on INLINEFORM2 is 3.6%. Our contributions in this paper are summarized below: The rest paper is organized as follows. In Section SECREF2 , we review the related work. In Section SECREF3 we provide an overview of ESIM (baseline) model and describe our methods to address out-of-vocabulary issues. In Section SECREF4 , we conduct extensive experiments to show the effectiveness of the proposed method. Finally we conclude with remarks and summarize our findings and outline future research directions.

Question: how does end of utterance and token tags affect the performance
Answer: Performance degrades if the tags are not used.