8. One or more hardware computer-readable memory devices or hardware storage devices comprising executable instructions which, when executed by one or more processors, cause the one or more processors to perform acts comprising: identifying data streams in different portions of a document associated with a query, 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; modeling 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, different from the first language model, of the second data stream obtained from the second portion of the document, the second language model reflecting 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; estimating mixture weights for combining the separate language models into a mixed language model; and creating the mixed language model by interpolating the mixture weights.
8. One or more hardware computer-readable memory devices or hardware storage devices comprising executable instructions which, when executed by one or more processors, cause the one or more processors to perform acts comprising: identifying data streams in different portions of a document associated with a query, 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; modeling 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, different from the first language model, of the second data stream obtained from the second portion of the document, the second language model reflecting 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; estimating mixture weights for combining the separate language models into a mixed language model; and creating the mixed language model by interpolating the mixture weights. 10. The one or more hardware computer-readable memory devices or hardware storage devices of claim 8 , the acts further comprising choosing query dependent mixture weights to optimize query likelihood in a ranking function that ranks the document.