1. A method of maintaining a search index, the method comprising: given a particular index that contains a plurality of entries, in which each entry of the plurality of entries identifies (a) a separate document from a first plurality of documents, (b) at least one data item that was excerpted from the document, (c) at least one category to which the at least one data item was deemed to belong, and (d) a measure of confidence that the at least one data item was correctly categorized as belonging to the at least one category: identifying, in two or more entries of the particular index, two or more particular data items that (a) are each associated, in the index, with a particular category, and (b) are each associated, in the index, with a measure of confidence of correct categorization that is at least as high as a specified confidence threshold; identifying one or more characteristics other than the particular category that are shared by each of the two or more particular data items; selecting, from the one or more characteristics, one or more particular characteristics; and for each particular document in a second plurality of documents, performing steps comprising: determining whether a candidate data item excerpted from the particular document has each of the one or more particular characteristics; and in response to a determination that the candidate data item has each of the one or more particular characteristics, inserting, into the particular index, an entry that identifies (a) the candidate data item and (b) the particular category, thereby indicating that the candidate data item belongs to the particular category.