1. A method of training a classifier system by utilizing previously classified data objects organized into a subject hierarchy of a plurality of nodes, the method comprising: selecting one node of the plurality of nodes; aggregating those of the previously classified data objects corresponding to the selected node and any associated sub-nodes of the selected node, to form a content class of data objects; aggregating those of the previously classified data objects corresponding to any associated sibling nodes of the selected node and any associated sub-nodes of the sibling nodes to form an anti-content class of data objects; and extracting features from at least one of the content class of data objects and the anti-content class of data objects to facilitate characterization of said previously classified data objects.