16. The system of claim 15 , wherein the segment inventory estimator comprises: a cluster deliverable inventory estimator that determines a deliverable inventory estimate for the segment for each cluster to generate a plurality of cluster-specific deliverable inventory estimates; and a summer that sums the plurality of cluster-specific deliverable inventory estimates to generate the estimate of the deliverable inventory for the segment.
16. The system of claim 15 , wherein the segment inventory estimator comprises: a cluster deliverable inventory estimator that determines a deliverable inventory estimate for the segment for each cluster to generate a plurality of cluster-specific deliverable inventory estimates; and a summer that sums the plurality of cluster-specific deliverable inventory estimates to generate the estimate of the deliverable inventory for the segment. 17. The system of claim 16 , wherein for each cluster, the cluster deliverable inventory estimator sums the probabilities of the search keywords included in the segment that are included in the plurality of keyword probability vectors for the cluster, and multiplies the summed probabilities for the cluster by the estimated total inventory for the cluster to determine the cluster-specific deliverable inventory estimate for the cluster.