Position of Navigation Elements
Where do users expect it?
From Bernard's (2001a,b) study of user expectations we find that users often expect to find navigation on the upper left side of a page. Bachiochi and his colleagues (1997) study of navigation items reveals that the best place for a sites main navigation items is the very top of the page near the browser controls (as I have done here). In a psychological study (Di Nocera, et al. 2004) cluster analysis of subjects choosing where they'd expect certain navigation links showed that there was a "natural deployment of web objects." They showed that semantically-based organization is suggested by their results. This natural positioning is related to the idea of affordance from Norman. Finally Nygren (1996) showed that scanning a web page and finding a particular target item was done quicker when the element were in familiar positions. Showing that continuity is important in web design.
Why is this particularly important?
One of the most important aspects of a website is the ability to navigate through it. What differentiates html from more traditional media is the ability to quickly traverse a large body of content via links. If the user is confused about a sites critical navigation links then they will easily become confused and disoriented, often leaving the site in frustration.
How does this interact with site structure?
The interaction here goes in both directions, limits on the visual presentation of the top level of site structure on a page limits the structure, and people's mental limits on the understanding of certain more complicated structures limits the design of the structure and thus what we see on the page. Particularly findings show that users can feel disoriented and perform slower if there is too many categories at the top level, while to deep a structure can be even more difficult to use (Larson & Czerwinski, 1998).