1. A method comprising: ordering a list of keywords from high activity to low activity, partitioning the list into at least two sets, a head partition including keywords with an activity level above a predefined threshold, a tail partition including the remainder of the keywords in the list; modeling the keywords in the head partition based on a set of variables; scoring the keywords in the head partition based on the modeling; and clustering head partition keywords with tail partition keywords having at least one common variable into at least one keyword cluster.