A method of providing advertising services selects a finite set of topics, and arranges the selected set of topics into a hierarchical structure. The method classifies impression items into the nodes within the hierarchical structure, and allows bidding against the nodes within the hierarchical structure. Some embodiments allow a bidder to request a refinement of the hierarchical structure. These embodiments receive such a request, and compare the request to a set of criteria. If the request meets the set of criteria, the method divides a first node in the hierarchical structure to at least a second and third node. The method allows bidders to bid on each of the first, second, and third nodes. The method optionally measures a performance for the nodes within the hierarchical structure. Based on the measure of performance for the nodes, the method preferably removes an under-performing node from the hierarchical structure.