1. A method performed by data processing device, the method comprising: obtaining, by a data processing device, search session data comprising search sessions, each search session including one or more search queries received by a search engine, wherein each search query comprises one or more words; determining, by a data processing device, a threshold out of vocabulary rate, the out of vocabulary rate indicating a rate at which a word included in a search query is not included in a vocabulary of words; determining, by a data processing device, a threshold session out of vocabulary rate, the session out of vocabulary rate indicating a rate at which search sessions have an out of vocabulary rate that meets the threshold out of vocabulary rate; and selecting, by a data processing device, a vocabulary of words that, for a set of test data, has a session out of vocabulary rate that meets the threshold session out of vocabulary rate, the vocabulary of words being selected from the one or more words included in each of the search queries included in the search sessions.