9. The method of claim 8 , wherein said estimating a deliverable inventory for the segment comprises: receiving an indication of a plurality of clusters; receiving a plurality of page view probability vectors that includes a page view probability vector for each viewed page of a plurality of previously viewed pages, each page view probability vector indicating probabilities of a corresponding viewed page being included in each of the clusters; determining a deliverable inventory estimate for the segment for each cluster to generate a plurality of cluster-specific deliverable inventory estimates; and summing the plurality of cluster-specific deliverable inventory estimates.
9. The method of claim 8 , wherein said estimating a deliverable inventory for the segment comprises: receiving an indication of a plurality of clusters; receiving a plurality of page view probability vectors that includes a page view probability vector for each viewed page of a plurality of previously viewed pages, each page view probability vector indicating probabilities of a corresponding viewed page being included in each of the clusters; determining a deliverable inventory estimate for the segment for each cluster to generate a plurality of cluster-specific deliverable inventory estimates; and summing the plurality of cluster-specific deliverable inventory estimates. 10. The method of claim 9 , wherein said determining a deliverable inventory estimate for the segment for each cluster to generate a plurality of cluster-specific deliverable inventory estimates comprises: receiving a plurality of keyword probability vectors that includes a keyword probability vector for each of a plurality of previously submitted search keywords, each keyword probability vector indicating probabilities of a corresponding keyword being included in each of the clusters; and for each cluster, summing the probabilities of the search keywords included in the segment that are included in the plurality of keyword probability vectors for the cluster, and multiplying 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.