9.9. PolyhierarchyIn a strict hierarchy, each term appears in one and only one place. This was the original plan for the biological taxonomy. Each species was supposed to fit neatly into one branch of the tree of life. kingdom: phylum: sub-phylum: class: order: family: species However, things didn't go according to plan. In fact, biologists have been arguing for decades over the correct placement of various species. Some organisms have the audacity to exhibit characteristics of multiple categories. If you're a purist, you can attempt to defend the ideal of strict hierarchy within your web site. Or, if you're pragmatic, you can allow for some level of polyhierarchy, permitting some terms to be cross-listed in multiple categories. This is shown in Figure 9-24. Figure 9-24. Hierarchy and polyhierarchyWhen you're dealing with large information systems, polyhierarchy is unavoidable. As the number of documents grows, you need a greater level of precoordination (using compound terms) to increase precision, which forces polyhierarchy. For example, Medline cross-lists viral pneumonia under both virus diseases and respiratory tract diseases (Figure 9-25). Figure 9-25. Polyhierarchy in MedlineYahoo! is another large site that makes prolific use of polyhierarchy (Figure 9-26). The @ signs are used to note categories that are cross-listed under other branches within the hierarchy. In the classification and placement of physical objects, polyhierarchy causes a problem. Physical objects can typically be in only one place at one time. The Library of Congress classification scheme was developed so that each book in a library could be placed (and found) in one and only one location on the shelves. In digital information systems, the only real challenge introduced by polyhierarchy is representing the navigational context. Most systems allow for the notion of primary and secondary locations within the hierarchy. Yahoo!'s @ signs lead users from the secondary to the primary locations. Figure 9-26. Polyhierarchy within Yahoo! |