15. A computer readable storage device encoded with a computer program, the program comprising instructions that when executed by a data processing device cause the data processing device to perform operations comprising: obtaining 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 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 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 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.
15. A computer readable storage device encoded with a computer program, the program comprising instructions that when executed by a data processing device cause the data processing device to perform operations comprising: obtaining 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 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 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 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. 21. The computer storage device of claim 15 , wherein the set of test data comprises second search session data that was obtained from a speech recognition system, the second search session data including one or more transcribed search queries, each transcribed search query comprising one or more words that were transcribed from spoken words that were received by the speech recognition system.