1. A computer-implemented method of generating data extraction rules suitable for extracting a datapoint from a set of markup language documents, the method comprising: generating a first extraction rule for extracting the datapoint from the set of markup language documents, wherein the first extraction rule is derived from, and includes extraction information obtained from, a first markup language document of the plurality of markup language documents; generating a second extraction rule for extracting the datapoint from the set of markup language documents, wherein the second extraction rule is derived from, and includes extraction information obtained from, a second markup language document of the plurality of markup language documents, wherein the first extraction rule and the second extraction rule are each capable of being applied independently of the other to extract the datapoint from documents in said set; and merging the first extraction rule with the second extraction rule to create a third extraction rule for extracting the datapoint from the set of markup language documents, wherein merging the first and second extraction rules comprises retaining extraction information common to, and discarding extraction information not common to, the first and second extraction rules, such that the third extraction rule reflects correlations between the first and second documents, said merging performed automatically by a computer.