Text Classification is one of the most common tasks in Natural Language Processing. When proposing new classification models, practitioners select a sample of items the proposed model classified correctly while the baseline did not, and then try to observe patterns across those items to understand the proposed model’s strengths. However, this approach is not comprehensive and requires the effort of observing patterns across text items. In this work, we propose a new evaluation methodology for performing qualitative assessment over multiple classification models. The proposed methodology is driven to discover clusters of text items where each cluster’s items 1) exhibit a linguistic pattern and 2) the proposed model significantly outperforms the baseline when classifying such items. This helps practitioners in learning what their proposed model is powerful at capturing in comparison with the baseline model without having to perform this process manually. We use a fine-tuned BERT and Logistic Regression as the two models to compare with Sentiment Analysis as the downstream task. We show how our proposed evaluation methodology discovers various clusters of text items which BERT classifies significantly more accurately than the Logistic Regression baseline, thus providing insight into what BERT is powerful at capturing.

Linguistic analyses in natural language processing (NLP) have often been performed around the static notion of words where the context (surrounding words) is not considered. For example, previous analyses on politeness have focused on comparing the use of static words such as personal pronouns across (im)polite requests without taking the context of those words into account. Current word embeddings in NLP do capture context and thus can be leveraged to enrich linguistic analyses. In this work, we introduce a model which leverages the pre-trained BERT model to cluster contextualized representations of a word based on (1) the context in which the word appears and (2) the labels of items the word occurs in. Using politeness as case study, this model is able to automatically discover interpretable, fine-grained context patterns of words, some of which align with existing theories on politeness. Our model further discovers novel finer-grained patterns associated with (im)polite language. For example, the word please can occur in impolite contexts that are predictable from BERT clustering. The approach proposed here is validated by showing that features based on fine-grained patterns inferred from the clustering improve over politeness-word baselines.