2. The system of claim 1 , wherein the operations further comprise: determining a context associated with the spoken query term; and selecting a subset of the past search queries that include contexts which are similar to the context of the spoken query term, wherein determining, for each n-gram, a frequency with which the n-gram occurs in the past search queries further comprises determining, for each n-gram, a frequency with which the n-gram occurs in the past search queries that are members of the subset only.
2. The system of claim 1 , wherein the operations further comprise: determining a context associated with the spoken query term; and selecting a subset of the past search queries that include contexts which are similar to the context of the spoken query term, wherein determining, for each n-gram, a frequency with which the n-gram occurs in the past search queries further comprises determining, for each n-gram, a frequency with which the n-gram occurs in the past search queries that are members of the subset only. 6. The system of claim 2 , wherein: determining a context associated with the spoken query term further comprise determining a location of the user when the spoken query term was spoken by the user; and selecting a subset of the past search queries that include contexts which are similar to the context of the spoken query term further comprises selecting a subset of the past search queries that were submitted by the user when the user was within a predetermined distance from the location, or when the user was in a same geographic region as the location.