1. A method comprising: receiving a collection of documents; for each document of the collection of documents: breaking the document into pieces of text, and extracting n+k-grams from each piece of text separately such that each n+k-gram does not overlap; using the extracted n+k-grams for each document as a collection of candidate phrases for a given topic; assigning weights to the candidate phrases; and generating a linear classifier using the weighted candidate phrases, wherein the linear classifier varies the weights for each phrase candidate depending on the length of a document being classified.