Do state-of-the-art natural language understanding models care about word order - one of the most important characteristics of a sequence? Not always! We found 75% to 90% of the correct predictions of BERT-based classifiers, trained on many GLUE tasks, remain constant after input words are randomly shuffled. Despite BERT embeddings are famously contextual, the contribution of each individual word to downstream tasks is almost unchanged even after the word's context is shuffled. BERT-based models are able to exploit superficial cues (e.g. the sentiment of keywords in sentiment analysis; or the word-wise similarity between sequence-pair inputs in natural language inference) to make correct decisions when tokens are arranged in random orders. Encouraging classifiers to capture word order information improves the performance on most GLUE tasks, SQuAD 2.0 and out-of-samples. Our work suggests that many GLUE tasks are not challenging machines to understand the meaning of a sentence.

此研究发现，BERT等自然语言处理模型在进行自然语言理解任务时，即使输入词的顺序被随机打乱，其对下游任务的影响几乎不会改变，因为这些模型主要利用表面特征（如情感分析中的关键词情感或自然语言推断中序列对输入的逐词相似性）进行正确的决策。研究结果表明，许多自然语言处理任务不能真正挑战机器对句子含义的理解能力。

乱序排列：语句中单词顺序对自然语言理解任务的重要性如何？