eage) has produced numerous relevant results here. Buneman et al. 8 devised a system for tracking user’s browsing and collection activities so they can be queryable later on. They accomplished this by supplementing the user’s personal database with a separate provenance database that links items to their original sources, and to previous versions within the local database. The Trio system39 also automatically tracks when and how data items came to exist, whether imported from outside sources, or computed from other known facts. This allows a history of each item to be reconstructed, and the database to be selectively filtered based on source or time information. Bhagwat et al. 5 specifically studied the propagation of annotations, so that as data evolves over time source data can be recovered. These techniques are applicable to PKB implementation as well, to enable users to browse and collect information and know that source information will automatically be tracked.

figure 5. ViKi, one of the first spatial hypertext systems. Rather than links between elements, the primary way organizational information is conveyed is through spatial clustering.

Data models

Kaplan et al. stated it well when they observed in 1990 that “dominant database management paradigms are not well suited for managing personal data,” since “personal information is too ad hoc and poorly structured to warrant putting it into a record-ori-ented online database.” 25 Clearly this is the case; when we want to jot down and preserve a book recommendation, directions to a restaurant, or scattered lecture notes, a rigidly structured relational database table is exactly the wrong prescription. The random information we collect defies categorization and quantization, and yet it demands some sort of structure, both to match the organized fashion in which we naturally think and to facilitate later retrieval. The question is, what sort of data model should a PKB provide?

A few definitions are in order. First,
we will use the term “knowledge ele-
ment” to refer to the basic building
blocks of information that a user cre-
ates and works with. Most systems
restrict knowledge elements to be
simple words, phrases, or concepts,
although some (especially note-taking
systems) permit larger blocks of free
text, which may even include hyper-
links to external documents. Second,
the term “structural framework” will
cover the rules about how these knowl-
edge elements can be structured and
interrelated.

the five Primary structural
frameworks
Tree. Systems that support a tree mod-
el allow knowledge elements to be or-
ganized into a containment hierarchy,
in which each element has one and
only one “parent.” This takes advan-
tage of the mind’s natural tendency to
classify objects into groups, and to fur-
ther break up each classification into
subclassifications.