This paper has discussed several of the parameters of variation in dynamic hypertext systems. All of these parameters are associated with kinds of flexibility which seem to be desirable in the exploitation of the dynamic hypertext concept, but they are not necessarily independent or all useful together. Indeed, flexibility is not without cost; for instance, canned text can offer fluency which the most advanced NLG systems today have yet to reach.
Figure 5 summarises our understanding of how the six systems mentioned earlier vary in what they provide. Obviously such a short table cannot do justice to the detailed contributions of these systems, however.
: Dimensions of Flexibility in Existing Systems
Some of the parameters are relatively well-understood, and the state-of-the-art in NLG is sufficient to allow us to manipulate them effectively in dynamic hypertext systems. However, other parameters---particularly those involving user freedom and system goals---require further investigation.
The domain we have been investigating raises the particular challenge of mixed initiative dialogue: the visitor is in control of macro-level content selection, but the system selects lower-level content, with the twin goals of satisfying the visitor's curiosity, and of conveying key information. In a sense, meeting the challenge subverts the very idea of hypertext as we know it.
In conventional hypertext, the author selects content and connectivity, and then retires; the user is free to sample the hyper-document however they wish. Because of the author's disappearance, the effectiveness of the sampling is crucial: authors strive to place links and important content in all the right places, but some users still feel that they have to sample exhaustively every node of the hypertext. Nonetheless, hypertext is attractive because users value their freedom to sample as they choose.
In a system which opportunistically both satisfies user curiosity and its own informational goals, we can provide an ersatz author. However the user samples, they will always end up with the important content. In a sense, the users' freedom is an illusion: they cannot fail. When fishing on an ordinary river, an angler casts their line, and occasionally, if they do it well enough, and wait long enough, they catch a fish. We are proposing a new kind of river, in which an angler can cast their line haphazardly, and still pull out a perfect fish, time after time.
