17. The computer storage device of claim 15 , wherein the set of test data comprises second search session data that was obtained during a second period of time that occurs subsequent to a first period of time during which the first search session data was received.
17. The computer storage device of claim 15 , wherein the set of test data comprises second search session data that was obtained during a second period of time that occurs subsequent to a first period of time during which the first search session data was received. 20. The computer storage device of claim 17 , wherein the vocabulary of words is selected based on an aggregate out of vocabulary rate for the test data, the aggregate out of vocabulary rate indicating a rate at which a word included in a test data is not included in the vocabulary of words.