Recent work has demonstrated that pre-trained language models (PLMs) are zero-shot learners. However, most existing zero-shot methods involve heavy human engineering or complicated self-training pipelines, hindering their application to new situations. In this work, we show that zero-shot text classification can be improved simply by clustering texts in the embedding spaces of PLMs. Specifically, we fit the unlabeled texts with a Bayesian Gaussian Mixture Model after initializing cluster positions and shapes using class names. Despite its simplicity, this approach achieves superior or comparable performance on both topic and sentiment classification datasets and outperforms prior works significantly on unbalanced datasets. We further explore the applicability of our clustering approach by evaluating it on 14 datasets with more diverse topics, text lengths, and numbers of classes. Our approach achieves an average of 20% absolute improvement over prompt-based zero-shot learning. Finally, we compare different PLM embedding spaces and find that texts are well-clustered by topics even if the PLM is not explicitly pre-trained to generate meaningful sentence embeddings. This work indicates that PLM embeddings can categorize texts without task-specific fine-tuning, thus providing a new way to analyze and utilize their knowledge and zero-shot learning ability.

本文通过将聚类方法应用于提前训练的语言模型的嵌入空间中，展示了在主题和情感分类数据集上，该方法在无需另作准备的情况下显著提高了零-shot文本分类的性能; 并发现该方法不需要Fine-tuning就可以将文本分类；最后，文章还比较了不同PLM嵌入空间，并发现即使PLM未明确预训练为生成有意义的句子嵌入，它仍然能按主题将文本很好地聚类。

通过聚类表示，使预训练语言模型成为更好的零样本学习器