1. A method of automatically generating transcoding instructions to locate and extract a subset of data from a selected web page of a web site, the method comprising: receiving an input describing the subset of data, said input comprising one or more data fields and, for each data field, respective field values from at least two sample web pages of a web page family for the web site; and for each data field: comparing respective web page code defining the respective field values for commonality to find a matching pattern with which to locate the respective field values, said matching pattern comprising a signature for the data field; and defining the transcoding instructions in accordance with the matching pattern to locate and extract field values for the data field within web pages of the web page family.