1. A method of indexing documents in a document collection, each document having an associated identifier, the method comprising: providing a list of phrases; identifying, by operation of a processor adapted to manipulate data within a computer system,  for a given document, phrases from the list of phrases that are present in the document; for each identified phrase in the document: identifying, by operation of a processor adapted to manipulate data within a computer system, a related phrase also present in the document, wherein for each phrase g j , g k is a related phrase of phrase g j where an information gain I of g k with respect to g j exceeds a predetermined threshold, the information gain I being a function of A(j,k) and E(j,k), where A(j,k) is a measure of an actual co-occurrence rate of g j and g k , and E(j,k) is an expected co-occurrence rate g j and g k ; and indexing, by operation of a processor adapted to manipulate data within a computer system, the document by storing the identifier of the document and an indication of each related phrase g k also present in the document, in a posting list of the identified phrase g j .