Bruza proposed a two-level architecture for hypertext
documents, the top level called hyperindex (containing index information)
and the bottom level hyperbase (containing content nodes and links) [Bruza,
1990]. The hyperindex consists of a set of indexes linked together.
When an index term describing the required information is found, the objects
from the underlying hyperbase are retrieved for examination. Navigating
through the hyperindex (not the hyperbase) and retrieving information
from the hyperbase is called "Query By Navigation" [Bruza,
An index is made of a set of index entries. Each index entry consists
of a term descriptor or keyword and a locator (like a page number). Term
descriptors lack specificity. Term phrases are made of term descriptors
thus increasing specificity. However, they may retrieve too many items
or no items at all and hence lack exhaustivity. Index expressions provide
relationships between term descriptors. Thus, they are more specific than
term phrase descriptors. Index expressions have a structure that can be
used to derive a lattice of descriptors supporting query by navigation.
A base index expression consists of terms that are linked to other terms
by connectors. For example, "effective information retrieval"
is a base index expression. So is "people in need of information".
The two combined together form an index expression. For example, "effective
information retrieval information AND people in need of information".
The power base expression is a lattice formed out of a full base expression
at the top and an empty base expression at the bottom. This lattice (or
lattice-like) structure is the basis of the hyperindex [Bruza,
1990]. Based on the vertex of focus in the lattice, the surrounding
descriptors can represent enlargements (context extension) or refinements
(context contraction) of the context represented by the focus. Thus, the
reader can move across the lattice by refining or enlarging the current
focus until a focus is found which is relevant to the information required.
Bruza uses some measures to determine the effectiveness
of index expressions in the hyperindex.