14. A system comprising: one or more processors; and computer-executable instructions that, when executed by the one or more processors, configure the one or more processors to: identify data streams in different portions of a document, the data streams including at least a first data stream obtained from a first portion of the document and a second data stream that is different from the first data stream, the second data stream being obtained from a second portion of the document other than the first portion; model the data streams as separate language models corresponding to the different portions of the document, the separate language models including at least: a first language model of the first data stream obtained from the first portion of the document, the first language model reflecting first language usage in the first portion of the document, and a second language model of the second data stream obtained from the second portion of the document, wherein the second language model is different than the first language model and reflects second language usage in the second portion of the document that is different from the first language usage in the first portion of the document; and use the separate language models to estimate a probability that the document matches a query by: adapting the separate language models individually to match the query, and characterizing the document as a mixture model that combines the separate language models.
14. A system comprising: one or more processors; and computer-executable instructions that, when executed by the one or more processors, configure the one or more processors to: identify data streams in different portions of a document, the data streams including at least a first data stream obtained from a first portion of the document and a second data stream that is different from the first data stream, the second data stream being obtained from a second portion of the document other than the first portion; model the data streams as separate language models corresponding to the different portions of the document, the separate language models including at least: a first language model of the first data stream obtained from the first portion of the document, the first language model reflecting first language usage in the first portion of the document, and a second language model of the second data stream obtained from the second portion of the document, wherein the second language model is different than the first language model and reflects second language usage in the second portion of the document that is different from the first language usage in the first portion of the document; and use the separate language models to estimate a probability that the document matches a query by: adapting the separate language models individually to match the query, and characterizing the document as a mixture model that combines the separate language models. 15. The system of claim 14 , wherein the computer-executable instructions further configure the one or more processors to: identify other data streams in other portions of other documents and model the other data streams; estimate other probabilities that the other documents match the query; and rank the document relative to the other documents based on the probability and the other probabilities.