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the study were expressed through the shallow hierarchies of photo sets and collections created by Flickr users to manage their photos. Our approach is general, and can be applied to other systems that allow users to specify relations: e.g., the social bookmarking site Del.icio.us allows users to group related tags into tag bundles.

Our long-term goal is to learn the structure of collective knowledge from the evidence provided by many users [16]. We believe that the simple relations described above are more informative than tags alone for learning how people classify things. Although we have not quantitatively compared the folksonomy learned by our approach to existing classification systems, qualitative evaluation indicates that our baseline method already yields good quality folksonomies. There is still much room for improvement. In the future, we plan to separate
"broader/narrower" from "related-to" relations. We also need to more systematically handle the challenges of different users using a different classification order and different level of specificity in the relations they specify. We would also like to combine relations with tag statistics to disambiguate concepts. We would also like to perform a systematic evaluation of the learned folksonomies, e.g., by comparing learned structures to ODP's dmoz, the open Web directory.