4. The method of claim 3 , wherein the vocabulary of words is selected based on a minimum freshness of the vocabulary of words that corresponds to the threshold session out of vocabulary rate, the freshness of the vocabulary of words indicating a length of time between an end of the first period of time and a beginning of the second period of time.
3. The method of claim 1 , 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.