Open World Classification with Adaptive Negative Samples

Open world classification is a task in natural language processing with key practical relevance and impact. Since the open or {\em unknown} category data only manifests in the inference phase, finding a model with a suitable decision boundary accommodating for the identification of known classes and discrimination of the open category is challenging. The performance of existing models is limited by the lack of effective open category data during the training stage or the lack of a good mechanism to learn appropriate decision boundaries. We propose an approach based on \underline{a}daptive \underline{n}egative \underline{s}amples (ANS) designed to generate effective synthetic open category samples in the training stage and without requiring any prior knowledge or external datasets. Empirically, we find a significant advantage in using auxiliary one-versus-rest binary classifiers, which effectively utilize the generated negative samples and avoid the complex threshold-seeking stage in previous works. Extensive experiments on three benchmark datasets show that ANS achieves significant improvements over state-of-the-art methods.

Ranking-Enhanced Unsupervised Sentence Representation Learning

Previous unsupervised sentence embedding studies have focused on data augmentation methods such as dropout masking and rule-based sentence transformation methods. However, these approaches have a limitation of controlling the fine-grained semantics of augmented views of a sentence. This results in inadequate supervision signals for capturing a semantic similarity of similar sentences. In this work, we found that using neighbor sentences enables capturing a more accurate semantic similarity between similar sentences. Based on this finding, we propose RankEncoder, which uses relations between an input sentence and sentences in a corpus for training unsupervised sentence encoders. We evaluate RankEncoder from three perspectives: 1) the semantic textual similarity performance, 2) the efficacy on similar sentence pairs, and 3) the universality of RankEncoder. Experimental results show that RankEncoder achieves 80.07% Spearman's correlation, a 1.1% absolute improvement compared to the previous state-of-the-art performance. The improvement is even more significant, a 1.73% improvement, on similar sentence pairs. Also, we demonstrate that RankEncoder is universally applicable to existing unsupervised sentence encoders.

An End-to-end Approach for Handling Unknown Slot Values in Dialogue State Tracking

We highlight a practical yet rarely discussed problem in dialogue state tracking (DST), namely handling unknown slot values. Previous approaches generally assume predefined candidate lists and thus are not designed to output unknown values, especially when the spoken language understanding (SLU) module is absent as in many end-to-end (E2E) systems. We describe in this paper an E2E architecture based on the pointer network (PtrNet) that can effectively extract unknown slot values while still obtains state-of-the-art accuracy on the standard DSTC2 benchmark. We also provide extensive empirical evidence to show that tracking unknown values can be challenging and our approach can bring significant improvement with the help of an effective feature dropout technique.