6. A system comprising: a machine-readable index that associates information items and sentence fragments, wherein the information items are each one of a date, an entity name, and a concept; and one or more computers programmed to perform operations in accordance with computer-readable instructions, the operations comprising: identifying a collection of paraphrase pairs from the machine-readable index; determining a frequency of occurrence value for a first paraphrase pair of the collection of paraphrase pairs, wherein the frequency of occurrence value embodies the frequency at which the first paraphrase pair appears in the collection; and adding the first paraphrase pair to a data collection based at least in part on the frequency of occurrence value meeting a criterion, wherein each paraphrase pair comprises a first paraphrase and a second paraphrase, the first paraphrase comprises a proper subset of the words in a first sentence fragment, the second paraphrase comprises a proper subset of the words in a second sentence fragment, and each of the first paraphrase and the second paraphrase in a paraphrase pair are in a same language, have a same or a similar meaning, and are not identical.
8. The system of claim 6 , further comprising: determining a second frequency of occurrence value for a second paraphrase pair; and adding the second paraphrase pair to the machine-readable index based at least in part on the frequency of occurrence value meeting the criterion.