In cross-lingual named entity recognition (NER), self-training is commonly used to bridge the linguistic gap by training on pseudolabeled target-language data.However, due to sub-optimal performance on target languages, the pseudo labels are often noisy and limit the overall performance.In this work, we aim to improve self-training for cross-lingual NER by combining representation learning and pseudo label refinement in one coherent framework.Our proposed method, namely ContProto mainly comprises two components: (1) contrastive self-training and (2) prototype-based pseudo-labeling.Our contrastive self-training facilitates span classification by separating clusters of different classes, and enhances crosslingual transferability by producing closelyaligned representations between the source and target language.Meanwhile, prototype-based pseudo-labeling effectively improves the accuracy of pseudo labels during training.We evaluate ContProto on multiple transfer pairs, and experimental results show our method brings in substantial improvements over current stateof-the-art methods. 1

Data augmentation is an effective solution to data scarcity in low-resource scenarios.However, when applied to token-level tasks such as NER, data augmentation methods often suffer from token-label misalignment, which leads to unsatsifactory performance.In this work, we propose Masked Entity Language Modeling (MELM) as a novel data augmentation framework for low-resource NER.To alleviate the token-label misalignment issue, we explicitly inject NER labels into sentence context, and thus the fine-tuned MELM is able to predict masked entity tokens by explicitly conditioning on their labels.Thereby, MELM generates high-quality augmented data with novel entities, which provides rich entity regularity knowledge and boosts NER performance.When training data from multiple languages are available, we also integrate MELM with codemixing for further improvement.We demonstrate the effectiveness of MELM on monolingual, cross-lingual and multilingual NER across various low-resource levels.Experimental results show that our MELM presents substantial improvement over the baseline methods. 1

Cross-lingual named entity recognition (NER) suffers from data scarcity in the target languages, especially under zero-shot settings.Existing translate-train or knowledge distillation methods attempt to bridge the language gap, but often introduce a high level of noise.To solve this problem, consistency training methods regularize the model to be robust towards perturbations on data or hidden states.However, such methods are likely to violate the consistency hypothesis, or mainly focus on coarse-grain consistency.We propose ConNER as a novel consistency training framework for cross-lingual NER, which comprises of: (1) translation-based consistency training on unlabeled target-language data, and (2) dropoutbased consistency training on labeled sourcelanguage data.ConNER effectively leverages unlabeled target-language data and alleviates overfitting on the source language to enhance the cross-lingual adaptability.Experimental results show our ConNER achieves consistent improvement over various baseline methods. 1