ACM Computing Surveys
31(4), December 1999,
http://www.acm.org/surveys/Formatting.html. Copyright ©
1999 by the Association for Computing Machinery, Inc. See the permissions statement below.
Where have you been from here?
David De Roure,
University of Southampton
Trails in Hypertext Systems
Multimedia Research Group
Highfield Lane, Southampton, SO17 1BJ, UK
email@example.com, firstname.lastname@example.org, email@example.com, firstname.lastname@example.org
People are confronted with ever growing amounts of information.
This makes it increasingly difficult to navigate vast information
spaces in order to find appropriate information. User trails
provide a mechanism that allows people not only to better manage
their personal information spaces but also to ask questions such
as "where do other people go from here?", "what else should I
read?" or "how did we come to that conclusion?". The fact that
trails are built with information about the users' browsing paths
and activities makes them well suited for collaborative
applications where users with similar interests are to be matched.
Categories and Subject Descriptors: H.5.4 [Hypertext/Hypermedia]:
H.5.3 [Group and Organization Interfaces]: Web-based Interaction
General Terms: Hypertext, Collaboration, Social Navigation
Additional Key Words and Phrases: User trails, open hypermedia system, software agents
Since the Greek hero Theseus found his way out of the
maze by fastening a thread to the entrance and unwinding it
on his way in, we are familiar with the concept of user trails as a
means for navigating unknown territories.
In hypertext, guided tours and trails are part of the repertoire
of concepts that can be used to assist users in navigating vast
amounts of information (for an overview of navigating hypertexts
see e.g. [Nielsen 1995]). As early as 1945,
[Bush 1945] envisaged trails (or paths as
they are called elsewhere [Shipman 1998],
[Zellweger 1989]) in order to provide a
mechanism for finding a user's personal information but also to
allow these trails to be available to other users. Bush also
envisaged a new profession, the trail blazers, "... those
who find delight in the task of establishing useful trails through
the enormous mass of the common record." [Bush 1945].
Both tours and trails are collections of hypertext objects (i.e.
nodes), similar to lists of bookmarks [Keller 1997]. Tours
usually support a hierarchical structure; trails on the other
hand, typically constitute a specific path a user has chosen in
exploring a set of nodes. Hence, backtracking could for instance
form part of a trail whereas it would not be part of a tour
(see [Bieber 1994] for a discussion of backtracking).
Interestingly both concepts use associations between different
pieces of information in order to provide the users with a
sequential view of the information space. This is somehow at
variance with the fundamental idea of hypertext, namely to support
non-linear access to information.
2 Applications of Tours and Trails
Though at first sight similar, guided tours and user trails suggest
themselves to different types of applications.
Tours are typically applied to scenarios where users are introduced to
an unfamiliar subject [Bieber 1997a],
[Trigg 1988]. In
particular, tours can help to guide novice users through a set of nodes.
This has several implications. For instance, in most cases tours are
authored by domain experts explicitly, e.g. by teachers for educational
applications [Shipman 1998]. Furthermore,
branching tours offer a choice of destinations. It is important that the
system indicates to users if they get off the track
Like tours, trails are first-class structures that are imposed
over a set of nodes and that can be named, edited or communicated to
others. Unlike tours however, trails have an activity associated
with the individual steps - the trail marks - that a user has taken.
This could be as simple as "traversing" a link; it could be any
activity applied to the node such as "printing it
out" [Chalmers 1998],
[Pikrakis 1998]; or it could even be a script
that is executed during the traversal
Another fundamental difference between tours and trails lies in the fact
that the latter ones are established by a set of users making their
way almost incidentally through the information space. Hence, trails
more resemble "beaten paths" than perfectly laid out walk ways.
This results in trails qualifying themselves for entirely different
application areas. Trails allow users to ask questions such as "who
else has been here?", "where do people go from here?" or also "how
did we come to that
[De Roure 1998],
[Trigg 1988]. Therefore, this
category of systems can be thought of as recommender
[Resnick 1997] in that they
support users in making their choices.
3 Issues and Research Questions
There a number of research issues related to trails in hypertext
systems. In this section we will give an overview.
The first issue concerns the acquisition of trail information. Using
trails results in defining the relevance of information, e.g. for
navigation, from the point of view of a particular set of people rather
than pre-defined categorisations
[Chalmers 1998]. This is opposed to
automatic resource discovery (see also Chakrabarti's paper in this special
issue [Chakrabarti 2000]).
A side effect of this is that by having people do the (subjective) ratings complex,
heterogeneous multimedia data can be handled more
easily [Chalmers 1998]. Furthermore,
recording a trail should be possible with minimal user effort at the
same time guaranteeing the user's privacy by allowing them to choose which trails to
publish [Hill 1997].
Secondly, the more accurate the system's recommendation to the
users needs to be, the more complex (and intelligent) the
processing will be. Therefore, different paradigms such as
software agents offer themselves for application in this
Another issue related in particular to trails concerns the activity
performed when creating a trail mark. Clearly, not only will the
individual activities be different but they will also be related to each
other. Rosenberg [Rosenberg 1996] proposes
a framework consisting of three basic types - acteme, episode and session -
which can be used for grouping the various activities to higher level
entities. Issues that arise include the question when to end a trail or
also, whether a user's backtracking should be recorded (it might be an
explicit indication to others not to follow a particular path).
The issue of coherence leads us to the more general question of
understanding hypertext documents and what role trails can play to
assist this process. One's own trail through a hypertext
will not always be of use to other people, especially in the absence of additional
meta information that provides coherence to the reader
However, if coherence is given, tours as well as trails can be considered a
metaphor that supports the understanding of documents, in particular when
combined with e.g. graphical overview maps
[Thüring 1995]. This
indicates that trail models should support a concept similar to
composite nodes, e.g. individual "hops" that would compose a
"trip" through a hypertext. Users could then view the various legs of
their journey before actually following a particular (sub-)trail.
Finally, not only do hypertexts change over time but so do the
users' trails. This might be due to changing interests, improved
knowledge of a domain, etc. This implies however, that the
importance of trails will change over time. Experience has shown
that changes in results stemming from growth of the trailbase were
[De Roure 2000]. As more
trails are created, the statistical profile of the trailbase
alters, and the results that a query returns will change. Possible
approaches to address this issue include degrading the relevance
of trails over time by using a resource based concept (see e.g.
[Moreau 1997]); or,
the use of a clustering algorithm to identify and track changing
interests [Crabtree 1998].
By that the importance of trails can be weighed over time.
Guided tours and trails are powerful concepts for assisting users
in navigating vast information spaces. While tours often serve the
purpose of introducing a subject, trails offer themselves to more
advanced applications. In particular, the fact that trails are
built with information about the users' browsing paths and
activities makes them well suited for collaborative applications
where users with similar interests are to be matched. Given that
privacy is provided, this will allow trail-blazers to not only
recommend related nodes but also to suggest users with
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