1. A computer-readable medium containing instructions for controlling a computer system to rank documents, by a method comprising: providing a sentence classifier to classify sentences into classifications of sentences; training a document rank classifier by representing each document of training data by the classifications of its sentences as determined by the sentence classifier and a rank of each document; representing a document by the classifications of its sentences as determined by the sentence classifier; and applying the trained document rank classifier to the representation of the document to determine the rank of the document.