A system and method of obtaining an importance ranking for a hierarchical collection of objects. The hierarchical collection of objects is represented as a tree containing a plurality of nodes, and each node to be ranked is represented as a respective leaf node of the tree. To obtain the ranking of the respective leaf nodes, the system and method locally ranks nodes contained in one or more sub-trees of the tree, in which each sub-tree has a depth equal to one. Next, the local rankings are effectively propagated up the tree, and the local rankings are aggregated at each level of the hierarchy, until a final importance ranking for the leaf nodes is obtained.