Investigating the effectiveness of Data Augmentation and Contrastive Learning for Named Entity Recognition

Data Augmentation (DA) and Contrastive Learning (CL) are widely used in NLP, but their potential for NER has not yet been investigated in detail. Existing work is mostly limited to zero- and few-shot scenarios where improvements over the baseline are easy to obtain. In this paper, we address this research gap by presenting a systematic evaluation of DA for NER on small, medium-sized and large datasets with coarse and fine-grained labels. We report results for a) DA only, b) DA in combination with supervised contrastive learning, and c) DA with transfer learning. Our results show that DA on its own fails to improve results over the baseline and that supervised CL works better on larger datasets while transfer learning is beneficial if the target dataset is very small. Finally, we investigate how contrastive learning affects the learned representations, based on dimensionality reduction and visualisation techniques, and show that CL mostly helps to separate named entities from non-entities.