In addition to representing documents as language models, we can also represent
the topic of the query as a language model. In this case, the intuition is that
the language model is a representation of the topic that the information seeker
had in mind when she was writing the query. This leads to three obvious possibilities
for retrieval models based on language models: one based on the probability
of generating the query text from a document language model, one based
on generating the document text from a query language model, and one based on
comparing the language models representing the query and document topics. In
the next two sections, we describe these retrieval models in more detail.