2. A computer-implemented method of determining top phrases of a limited document collection, comprising: determining, by at least one processor of a computer system, top phrases for a plurality of documents in the limited document collection, wherein determining the top phrases of a document includes: identifying phrases in the document; for each identified phrase in the document, determining an importance score for the identified phrase based on occurrences of related phrases of the identified phrase, which are also in the document; associating each top phrase with the importance score for the document; for each top phrase, determining, by at least one processor of the computer system, an aggregate score of the top phrase for the limited document collection based on the top phrase's scores for individual documents of the limited document collection in which the top phrase appears; and selecting, by at least one processor of the computer system, a set of top phrases with the highest aggregate scores.
2. A computer-implemented method of determining top phrases of a limited document collection, comprising: determining, by at least one processor of a computer system, top phrases for a plurality of documents in the limited document collection, wherein determining the top phrases of a document includes: identifying phrases in the document; for each identified phrase in the document, determining an importance score for the identified phrase based on occurrences of related phrases of the identified phrase, which are also in the document; associating each top phrase with the importance score for the document; for each top phrase, determining, by at least one processor of the computer system, an aggregate score of the top phrase for the limited document collection based on the top phrase's scores for individual documents of the limited document collection in which the top phrase appears; and selecting, by at least one processor of the computer system, a set of top phrases with the highest aggregate scores. 5. The method of claim 2 , wherein determining an aggregate score for the top phrase comprises summing the scores of the top phrase for each document for which the top phrase is a top phrase.