19. The system of claim 12 , wherein selecting the candidate phrase for inclusion in the extracted set based on the combined score and based on the document phrase scores includes: selecting the candidate phrase when the number of documents for which the candidate phrase had at least a minimum document phrase score exceeds a third threshold.
12. A system for extracting a set of phrases from a plurality of documents, the system comprising: one or more computer readable media comprising executable instructions; and one or more processors configured to execute the instructions, wherein execution of the instructions causes the system to: for each document: identify a plurality of candidate phrases occurring in the document, wherein a candidate phrase includes two or more consecutive words that are determined to occur in the document; score candidate phrases in the document to produce respective document phrase scores for the candidate phrases for the document, the document phrase score for a candidate phrase being based on attributes of individual occurrences of the candidate phrase in the document, with at least some candidate phrases appearing repeatedly having a higher document phrase score than candidate phrases appearing once, and for a candidate phrase of the plurality of the candidate phrases: create, via the one or more processors, a combined score for the candidate phrase based on a plurality of different document phrase scores for the candidate phrase for respective different documents; and selecting the candidate phrase for inclusion in the extracted set based on the combined score for the candidate phrase and based on the document phrase scores for the candidate phrase.