27. A computer-readable medium having instructions stored thereon, which, when executed by a processor, cause the processor to perform operations comprising: receiving a query and a current version of a document; receiving quality of result data for a plurality of versions of the document and the query, the quality of result data specifying a respective version-specific quality of result statistic for each of the versions of the document with respect to the query; calculating a weight for the version-specific quality of result statistics corresponding to each version of the document, wherein the weight for a particular version of the document is determined at least in part on an estimate of a difference between the particular version and the current version of the document, and wherein calculating the weight for a particular version of the document comprises: obtaining a representation of the particular version of the document, wherein the representation is a first time distribution of shingles, calculating a difference score by comparing the first time distribution of shingles representing the particular version of the document to a second time distribution of shingles representing the current version of the document, wherein each shingle is a contiguous subsequence of one or more tokens in the document, and wherein each shingle is associated with a particular time that the shingle is first observed in a version of the document such that a distribution of the times associated with the shingles in a version of the document corresponds to the representation of the version of the document, and using the difference score to calculate a corresponding weight for the particular version of the document; determining a weighted overall quality of result statistic for the document with respect to the query, wherein determining the weighted overall quality of result statistic comprises weighting each version-specific quality of result statistic with the calculated weight and combining the weighted version-specific quality of result statistics; and associating the weighted overall quality of result statistic with the document.
27. A computer-readable medium having instructions stored thereon, which, when executed by a processor, cause the processor to perform operations comprising: receiving a query and a current version of a document; receiving quality of result data for a plurality of versions of the document and the query, the quality of result data specifying a respective version-specific quality of result statistic for each of the versions of the document with respect to the query; calculating a weight for the version-specific quality of result statistics corresponding to each version of the document, wherein the weight for a particular version of the document is determined at least in part on an estimate of a difference between the particular version and the current version of the document, and wherein calculating the weight for a particular version of the document comprises: obtaining a representation of the particular version of the document, wherein the representation is a first time distribution of shingles, calculating a difference score by comparing the first time distribution of shingles representing the particular version of the document to a second time distribution of shingles representing the current version of the document, wherein each shingle is a contiguous subsequence of one or more tokens in the document, and wherein each shingle is associated with a particular time that the shingle is first observed in a version of the document such that a distribution of the times associated with the shingles in a version of the document corresponds to the representation of the version of the document, and using the difference score to calculate a corresponding weight for the particular version of the document; determining a weighted overall quality of result statistic for the document with respect to the query, wherein determining the weighted overall quality of result statistic comprises weighting each version-specific quality of result statistic with the calculated weight and combining the weighted version-specific quality of result statistics; and associating the weighted overall quality of result statistic with the document. 39. The computer-readable medium of claim 27 , wherein calculating the weight further comprises calculating a number of times the document has changed as compared to a reference version of the document or an amount of time the reference version of the document was unchanged.