1. A computer-implemented method, comprising: under control of one or more computing systems configured with executable instructions, identifying a corpus of items, wherein a plurality of the corpus of items include annotations; processing the annotations to identify a plurality of non-random unit groups; for each of the plurality of non-random unit groups, determining a discount factor based at least in part on a quotient of a number of times the non-random unit group appears in the corpus and a number of items in the corpus; and generating a unit group dictionary that includes non-random unit groups with a discount factor above a defined threshold.