DOCUMENTATION NOTE QUERY‐DOCUMENT SYMMETRY AND DUAL MODELS

The idea that there is some natural symmetry between queries and documents is explored. If symmetry can be assumed, then it leads to a conception of ‘dual’ models in information retrieval (given a model, we can construct a dual model in which the roles of documents and queries are reversed). But symmetry breaks down in various ways, which may invalidate this construction. If we can construct a dual, it is not obvious that it can be combined with the original.