1. A system comprising: a machine-readable index; one or more computers programmed to perform operations in accordance with computer-readable instructions, the operations comprising: identifying, in the machine-readable index, a first sentence fragment and a second sentence fragment that are both associated with a same first information item, wherein the first information item is one of a date, an entity name, and a concept, wherein the machine-readable index comprises a plurality of information items and sentence fragments associated with respective of the information items; in response to identifying that the first sentence fragment and the second sentence fragment are both associated with a same first information item, identifying a paraphrase pair in the first and second sentence fragments; repeating the identifying of the first sentence fragment and the second sentence fragment and the identifying of the paraphrase pair to identify a plurality of paraphrase pairs; and determining a frequency of occurrence value for each of the paraphrase pairs in the plurality of paraphrase pairs, wherein the frequency of occurrence value embodies the frequency at which each paraphrase pair appears in the plurality of paraphrase pairs, wherein the paraphrase pair comprises a first paraphrase and a second paraphrase, the first paraphrase comprises a proper subset of the words in the first sentence fragment, the second paraphrase comprises a proper subset of the words in the second sentence fragment, and the first paraphrase and the second paraphrase are in a same language, have a same or a similar meaning, and are not identical.