Question: Why do they choose GMM as a clustering approach?
Evidence: We encode multi-domain data at the sentence level into vector representations. We then cluster these vector representations for each model using a Gaussian Mixture Model (GMM) with $k$ pre-defined clusters. We chose GMM as our clustering approach as it allows soft assignments (vs. hard assignments as in e.g. K-means) which we think fits the task better (as a sentence can be seen as drawn from a mixture of several domain). In all cases, to create a sentence representation we perform average pooling of the last hidden state (before the softmax layer) for each token in the sentence. To accelerate the clustering process and enable visualization we also experiment with performing dimensionality reduction with PCA over the sentence vectors before clustering them. We experiment with k in 5, 10 and 15 to test how adding flexibility would improve the domain clustering accuracy.
We propose two methods for domain data selection with pretrained language models.
Domain-Cosine In this method we first compute a query vector, which is the element-wise average over the vector representations of the sentences in the small in-domain set. We use the same sentence-level average-pooling approach as described in Section SECREF2 to obtain sentence representations. We then retrieve the most relevant sentences in the training set by computing the cosine similarity of each sentence with this query vector and ranking the sentences accordingly.
Domain-Finetune It is now common knowledge that pretrained language models are especially useful when fine-tuned for the task of interest in an end-to-end manner BIBREF41. In this method we fine-tune the pretrained LM for binary classification, where we use the in-domain sentences as positive examples, and randomly sampled general-domain sentences as negative examples. We then apply this classifier on the general-domain data and pick the sentences that are classified as positive as in-domain, or choose the top-k sentences as ranked by the classifier output distribution. This can be seen as an instance of positive-unlabeled learning for document-set expansion; see BIBREF42 for a recent discussion and methodology for this task.
Negative Sampling with Pre-ranking One problem that may rise when randomly sampling negative examples is that unlabeled in-domain sentences from the general-domain data may be sampled as negative examples – deteriorating the classifier performance. To alleviate this issue, we perform a biased sampling of negative examples. We first rank the general-domain data using the Domain-Cosine method, and then sample negative examples under a certain threshold in the ranking (in our experiments we sampled from the bottom two-thirds). Table TABREF31 shows an ablation for such pre-ranking, measuring precision, recall and F1 for binary classification on a held-out set for each domain. When not using pre-ranking, as the training data for the domain is larger, the precision is lower – since more in-domain examples are drawn as negative samples. Using pre-ranking indeed alleviates this issue, achieving higher F1 scores in all cases. Given the results in Table TABREF31 we always use pre-ranking in the following experiments.
We chose GMM as our clustering approach as it allows soft assignments which we think fits the task better seen as drawn from a mixture of several domain.
Answer: Because it allows soft assignments which they think fits the task better.