Estimating Embedding Vectors for Queries.

The dense vector representation of vocabulary terms, also known as word embeddings, have been shown to be highly effective in many natural language processing tasks. Word embeddings have recently begun to be studied in a number of information retrieval (IR) tasks. One of the main steps in leveraging word embeddings for IR tasks is to estimate the embedding vectors of queries. This is a challenging task, since queries are not always available during the training phase of word embedding vectors. Previous work has considered the average or sum of embedding vectors of all query terms (AWE) to model the query embedding vectors, but no theoretical justification has been presented for such a model. In this paper, we propose a theoretical framework for estimating query embedding vectors based on the individual embedding vectors of vocabulary terms. We then provide a number of different implementations of this framework and show that the AWE method is a special case of the proposed framework. We also introduce pseudo query vectors, the query embedding vectors estimated using pseudo-relevant documents. We further extrinsically evaluate the proposed methods using two well-known IR tasks: query expansion and query classification. The estimated query embedding vectors are evaluated via query expansion experiments over three newswire and web TREC collections as well as query classification experiments over the KDD Cup 2005 test set. The experiments show that the introduced pseudo query vectors significantly outperform the AWE method.