Accessibility of an interface is dependent on a number of attributes of its presentation hierarchy. Access costs on the links, of course, is the primary measure. Capacity of a composite node determines the gain in terms of the number of pieces of information one would obtain by accessing a composite node. Thus, access cost along a path multiplied by the capacities of a composite nodes may give a good measure of accessibility. However, one should not assume that access is strictly hierarchical by accessing successive composite nodes. The layout function at composite nodes may allow contents to be visible and accessible at many different levels simultaneously, thus the visible depth of the presentation is a determining factor as well. The layout function at each composite node determines the visibility and dynamics of its components, thus it has an important impact on accessibility.
The layout function of the WebBook and the Web Forager allows shuffling of pages in a book for rapid access to a page of interest. However, still the access is linear in terms of the number of pages in the book. While the bookmarks on the book allow shortcut access to a few pages, it is still a linear access for most of the pages. The layout function of the shelves and the desk allows a two-dimensional organization of the books which allows direct access, thus access is independent of the number of books. However, one should notice that both have a certain capacity, limiting the accessibility. The visible depth, thus the access is limited to only three levels.
The layout function of the vtwm interface allows the user to select a desktop and access to all its windows, thus providing good accessibility. However, the depth of the presentation hierarchy is 3, thus limiting accessibility.
In the Elastic Windows interface the depth of the presentation hierarchy is unlimited. Multiple levels of the hierarchy are visible simultaneously, thus users can rapidly access windows at any level of the hierarchy by skipping intermediary levels. This allows access to pages in less than linear time depending on the capacity of window nodes. Thus, Elastic Windows interface provides fairly good accessibility.