An apparatus comprises a reduction tree to rank a given item of a set of M items relative to other items of the set of M items, in dependence on ranking information indicating an order of preference for the set of M items. The reduction tree has a number of levels of node circuits arranged in a tree structure, each node circuit configured to generate a plurality of node output signals indicative of whether a corresponding subset of the set of M items includes at least N more preferred items than the given item, where N≥2. A node circuit at a level of the reduction tree other than a first level is configured to combine the node output signals generated by at least two node circuits at a previous level of the reduction tree, such that the number of items in the corresponding subset increases through successive levels of the reduction tree, until the subset of items corresponding to a root node circuit at a final level of the reduction tree comprises the set of M items.