Usable Browse Hierarchies
Notes on Toward Usable Browse Hierarchies for the Web (.doc) by Kirsten Risden of Microsoft Research (1999). Looking at Yahoo-style category and sampling of sub-category navigation, she seems to suggest that using polyhierarchies are a way of compensating for ambiguous categories and labels. Well yes, one might say, but if it helped people find what they're looking for, isn't this just an academic argument? But the point is well taken: if you're turning to polyhierarchies to solve a findability issue than perhaps your categories (or any categories) aren't the solution.