Proceedings of the 2nd Workshop on Adaptive Systems and User Modeling on the WWW

Design Issues in Adaptive Web-Site Development

Paul De Bra [*]
Eindhoven University of Technology
The Netherlands

Abstract: For almost a decade people have been developing hypertext or hypermedia applications that adapt to some "features" of their users, like knowledge or preferences [Brusilovsky, 1996]. Recently some adaptive application environments have become available that use World Wide Web technology. Examples of such systems are Interbook [Brusilovsky et al., 1998] and AHA [De Bra & Calvi, 1998]. The adaptation can range from a simple (automatic) selection between different versions of some information pages to the completely dynamic generation of all pages from atomic information units and the automatic generation of all hypertext links. This paper sketches a general architecture for adaptive Web-sites by building on existing models such as Dexter [Halasz & Schwartz, 1994] and IMMPS [Bordegoni et al., 1997].

More importantly, this paper identifies issues in adaptive Web-site design for which no general approach or solution appears to exist (yet). These include (but are not limited to): the separation of a conceptual representation of an application domain from the content of the actual Web-site, the separation of content from adaptation issues, the structure and granularity of user models, the role of a user and application context, and the communication between different adaptive Web-site "engines".


Hypermedia systems in general, and Web-based systems in particular, are becoming increasingly popular as tools for user-driven access to information. The linking mechanism of hypermedia offers users a large amount of navigational freedom. Unfortunately, because of this freedom it becomes impossible for authors to anticipate all possible navigation paths a user can take. When a user (re-)visits a Web-site, she will often find links to information that is either not relevant for her current task or that is hard or impossible to understand. The user may be missing some important background knowledge or information that is available on the Web-site, but that is not necessarily visited first.

During the past decade different types of hypermedia systems and Web-sites were built that are able to perform some kind of personalization. There are different names for such systems or applications:

This paper focusses on Web-based adaptive hypermedia applications. This means that we assume that a hyperdocument exists that consists of (HTML) pages and links. An Adaptive Hypermedia System (AHS) or engine may change the content and presentation of nodes and may alter the link structure or annotate links, based on a user model. Such functionality can be achieved through some "standard" Web technology such as CGI-scripts, Java Servlets, or Active Server Pages. The aim of an adaptive Web-site is twofold: (Brusilovsky [Brusilovsky, 1996] talks about adaptive navigation support and adaptive presentation. However, we avoid these terms because they are confusing. Often the way in which navigation support is made adaptive is by means of link (anchor) annotation. In that case the "presentation" of the link is changed, but in [Brusilovsky, 1996] this is not considered to be adaptive presentation.)

This paper describes several problem areas related to adaptive hypermedia in general, and to adaptive Web-sites in particular. It is aimed at spawning some discussion on these issues, not on describing solutions that are widely accepted and used. Section 2 describes how a domain model can be designed such that information content and link structure can be described on a conceptual level and on a concrete (information) level. Section 3 focusses on user modeling and on how to provide adaptation (based on a user model) in such a way that the desired adaptation is easy to describe. It also introduces the notion of context, and shows how context relates to the domain model and the user model. Section 4 shows how adaptive hypermedia applications can be built using some "standard" Web-technology. Section 5 focusses on the communication between adaptive hypermedia systems. Before the use of adaptive hypermedia on the Web can become widespread we need an easy and flexible way to initialize an adaptive Web-site by importing user models from other sites (or other parts of the same Web-site). Different (sub-)applications may work together to build a common user model.

2. Modeling an Application Domain

An adaptive hypermedia application or Web-site deals with a certain subject domain. The description of this domain can be viewed (and described) at three levels:

When designing an adaptive application one has to decide which kinds of concept relationships need to be supported, and how these will be used. Most AHS support a fixed set of concept relationship types, such as hypertext links and prerequisite relationships.

An AHS should offer authors a tool to verify whether the generated concept relationship structure is sound. In an AHS with links and prerequisites this means that it must be possible to reach every page (from an author-defined starting point) without "violating" any prerequisite relationships. Note that this check involves links and prerequisites together. It implies the following rules (but is stronger than both of them):

One can think of other types of concept relationships as well. An example are inhibitor relationships: when A inhibits B this means that after a user has studied concept A she should not (or no longer) visit B. There are two approaches towards providing more flexibility in defining new kinds of relationships:

3. User Modeling and Adaptation

In order to create a Web-site that adapts itself to each individual user the server must register each user's action and deduce from that how the user's "state of mind" evolves. Based on this abstraction of the user's state the system can decide how to perform some adaptation. The representation of the user's state of mind is called a user model. It contains aspects that are controlled explicitly by the user, such as color or media preferences, learning style, background knowledge, job situation and other items that can be entered through a questionnaire. The more interesting part of a user model is the information the system maintains about the user's "relation" to the domain concepts. Furthermore, the system gathers this information by observing the user's browsing behavior.

The AHAM model [De Bra et al., 1999] describes the structure of a user model as a table that contains for each (domain model) concept a set of attribute/value pairs. The table below shows a very small example of such a user model.

name knowledge read ready
WWW learnt false false
HTML well-learnt true true
HTTP not-known false true
... ... ... ..

Each AHS may provide different attributes and different attribute types. An advanced AHS might even let the author or designer declare new attributes, provide rules to generate values for these attributes, and rules to perform adaptation based on these values. Some attributes typically found in AHS are:

Many other attributes can be thought of, including a knowledge decay value (how much the user has forgotten about this concept), an expiration time value (for pages with a dynamic content that is updated at certain time intervals, such as news bulletins or a weather forecast). Again, while having more attributes increases the possibilities for adaptation, it becomes more difficult to determine meaningful values for these attributes, and also to come up with meaningful rules to determine how the attributes must be used in the adaptation process.

There is a close tie between the value domain for the knowledge value attribute and the granularity of concepts. In the AHA system every page is considered a concept. Such fine grained approach is needed because the knowledge of a concept can only be true or false. In Pilar Da Silva's system the knowledge value ranges between 0 and 100. Several pages can contribute towards the knowledge of a single (higher-level) concept. Thus, a richer value domain for knowledge values enables a coarser granularity in the concept space. Such a simpler concept space is easier to design, and concept relationships can be simpler as well. (In AHA a concept relationship often involves a large number of concepts, as is exemplified in the hypermedia course 2L690.

Figure 2 shows the architecture of the IMMPS reference model for Intelligent Multimedia Presentations [Bordegoni et al., 1997]. The left-hand side shows the processes involved in handling a simple user interaction such as clicking on a link (which is a primitive way of formulating a goal). The right-hand side shows functional parts of the AHS that are involved in maintaining different models (domain model, user model, context model) and in designing presentations. We only describe the role of the processes on the left-hand side of the figure.

  1. First there is the Control Layer, in which the link (URL) is resolved. (This is part of the run-time layer of the Dexter [Halasz & Schwartz, 1994] and AHAM [De Bra et al., 1999] models.) This means that when the link does not identify a single Web-page but rather a composite concept, the AHS must deduce (from the domain model and the user model) which Web-page to show.
  2. Next the appropriate content is selected and retrieved in the Content Layer. (This corresponds to the accessor function in the Dexter (AHAM) model. The content is retrieved from the within-component layer.) In a Web-based system the content is a Web-page. The Web-page may be generated or assembled from fragments. The selection of the appropriate fragments is performed in this layer.
  3. The Design Layer is responsible for assembling a (Web-)page from the selected fragments. Fragments may need to be sorted, and links may be assigned to different link classes in order to make it possible to do link-annotation. The IMMPS model distinguishes three possible ways to achieve a properly designed Web-page that satisfies user-specific requirements as well as platform-specific requirements: layout after production, layout before production and layout interleaved with production. Apart from adaptation based on the user model the AHS must also adapt the Web-page to the user-interface (Web-browser) that will be used to view the page. The actual HTML code sent to a computer with a high resolution screen will be different from that sent to a Web-TV or to a PDA for instance.
  4. The Realization Layer is responsible for finalizing the Web-page so that it can be displayed by the browser. One of the tasks of this layer is to add the appropriate style sheet. This determines page-layout aspects for the content (fonts, alignment, etc.) as well as for the links (link color to class association for instance). (The result of this layer is what Dexter and AHAM call a presentation specification.)
  5. The Presentation Display Layer represents the rendering of the actual page, as performed by the Web-browser. This corresponds to the run-time layer of the Dexter (and AHAM) model.

The techniques for content-adaptation may be straightforward, and will not be elaborated upon in this section. We briefly describe different ways to perform link adaptation, and how they fit in with the model(s). (In Section 4 we describe how to implement content- and link-adaptation on the Web.)

In Section 4 we show how all these techniques can be realized using Web technology. Unfortunately we know of no current AHS that supports all these techniques (although each technique is supported in some existing AHS).

4. Realizing Adaptive Hypermedia on the Web

This section describes techniques that are available to develop adaptive Web-sites. It focusses on the use of Web-related technology, not on proprietary architectures. We separate issues related to domain-modeling, user-modeling, and the performing the actual adaptation. We illustrate how these aspects are handled in the AHA system [De Bra & Calvi, 1998] which is used for some Web-based courseware and a "kiosk" system at the Eindhoven University of Technology.

4.1 User-Modeling on the Web

In order to adapt to each individual user the AHS must maintain a model (representation) of the user's "state of mind". We are interested mainly in adaptation to individual users, not to groups. This suggests that the user model could well be stored on the user's (client) computer, and not on the Web-server. The concept of cookies was invented specifically for maintaining some user-dependent data on the client site. (See for details on cookies.) However there are some good reasons for maintaining the user model on the server side (or for not keeping it on the client side):

For the above (and possibly other) reasons existing Web-based AHS like AHA [De Bra & Calvi, 1998] and Interbook [Brusilovsky et al., 1998] store the user-model on the server-side.

Updates to the user-model can only be the result of a user action that involves an interaction with the Web-server. In systems like Interbook and AHA this happens whenever the user requests a page (by clicking on a link anchor) or when the user completes a form, like in the case of a (multiple choice) test. In AHA a special invisible "stop" applet is inserted in every page. This applet sends a request to the server each time a page is unloaded (because another page is accessed). AHA thus registers how long a user has been reading a page (or actually, how long the user's browser has been displaying the page). Using Dynamic HTML (and scripting languages like JavaScript) it is becoming possible to have the browser send requests to the server as a result of user actions without changing the page that is being displayed. This is interesting for user actions that cause additional information to become visible or that cause information to disappear. (Unfortunately it is not (yet) possible to associate scripting code to the scrolling event.

The user model in the AHA system consists of the following:

In the current version quite a bit of this information is not (yet) used: the browsing history is used to decide whether to color links blue or purple (or other colors if the user changes the preferences). Exact access and reading times are not used. Also, the knowledge of a concept is a Boolean value. In a future version we will be using a "percentage".

As the IMMPS model shows (see Figure 2) some designers wish to define a context model (or context expert) as well as a user model. Such a context model could maintain state information not directly related to the user, but related to the environment in which the application is being used. Examples of context information include properties of the computing environment of the user (e.g. screen resolution, network bandwidth) or of the application domain (e.g. the status of some machinery in a factory, the date or time, the location of the user, the current weather conditions, etc. Such contextual elements can be taken into account much in the same way as color preferences, knowledge about concepts or the outcome of multiple-choice tests. We therefore do not describe how to handle such a context model in a way that would differ from how the user model is handled.

4.2 Concepts and Content on an adaptive Web-site

In Section 3 we have proposed an architecture in which an application domain is described at three levels: concept, page and fragment. Since on the Web the "unit of presentation" is a (Web-)page, these three levels have to somehow be converted into one. Some ways in which this can be done are:

4.3 Techniques used to achieve Adaptive Content and Linking

There are two areas in which adaptive content is being used in AHS:

The presentation of a page, as seen by the user, is always the result of a filter operation performed by some server-side program. Such a program can be a CGI-script, a server plug-in or a Java Servlet. In case the presentation consists of several frames or "sub-windows", the content of each frame is generated by a separate request to the server and thus by a separate (run of the) program. Interbook uses a fixed presentation that consists of five frames. The overhead of executing five CGI-scripts is likely to be noticeable. The presentation in AHA is not fixed. An author can create any desired frames structure. AHA has been used for a course without the use of frames and for another course and a kiosk system, both of which use frames. The overhead in AHA is minimal thanks to the use of a Java-based Web-server together with Java Servlets.

Some kinds of link adaptation can be realized through adaptive content. In AHA, a link can be easily disabled and hidden through the inclusion of a conditional fragment:

   This course ends with an
   <!-- if ready-for-assignment -->
   <A HREF="assignment.html">assignment</A>
   <!-- else -->
   <!-- endif -- >
   that is accessible when you have studied all concepts.
However, most link-adaptation is performed by visually annotating link anchors to indicate whether the user is advised to follow the link or not.

In order to motivate more authors to use adaptive hypermedia the authoring process should be made much simpler than it is today. For Interbook the author must write MS-Word files that are structured in a specific way and that use (hidden) comments to provide information about the underlying concepts. For AHA the author must write HTML files with structured comments to provide information about concepts and to conditionally include fragments. We are currently working on a redesign of AHA that includes tools to facilitate authors in designing the concept space separately from the content space.

5. Communication Between Adaptive (Sub-)Applications

In order to perform adaptation to each user in a proper way an AHS needs to observe a user a long time. Currently each adaptive Web-site must start its observations from scratch. When adaptive sites can exchange information about the same user they can adapt to a user more quickly and in a better way. Therefore adaptive Web-sites should be able to exchange (parts of) user models.

Technically it is not difficult to enable adaptive Web-sites to exchange user model information. The AHA system offers a forms-based interface to the user model. Through that interface a user can view her user model, adjust it if desired, and submit it back to the server. It is easy to add a similar interface and API to enable remote systems to request a user model from AHA and to update it. (There is of course an issue of authorization that we ignore in this paper.) The still unsolved issue in communication between AHS in general (and adaptive Web-sites as well) is the semantics of the data in a user model:

It is easy to see that a general resolution for these issues cannot exist. Different universities (especially in different countries) do not agree on the issue which course from one institute is equivalent (or even better) than a course from the other institute. Different governments do not agree on the equivalence of university degrees.

Because of the ambiguities described above some conversion tools need to be built that are easily configured to translate concept names and to convert knowledge values. An area where standardization may help is in domain-independent aspects of a user model. This includes aspects like

6. Conclusions

Adaptive hypermedia has been around for about a decade. The Web offers the technological base for implementing most of the adaptive technology that has been implemented on other platforms. Some Web-based systems, including Interbook [Brusilovsky et al., 1998] and AHA [De Bra & Calvi, 1998] are being used, mostly in educational environments.

Authoring adaptive hypermedia remains a problem area, whether we are considering Web-based systems or not. Systems like Interbook and AHA were designed and implemented by computer scientists and the first applications of these systems were developed by (the same) computer scientists as well. Authoring is probably still too complicated for "average" authors from non-computer-related fields.

Web-based systems are expected to take advantage of the global nature of the Web. However, sharing user models still proves to be difficult because application domains are difficult to compare. Also, the internal representation of knowledge about an application domain in one system is difficult to translate to the internal representation of the same application domain in another adaptive hypermedia system.

7. References

[Bordegoni et al., 1997]
M. Bordegoni, G. Faconti, S. Feiner, M.T. Maybury, T. Rist, S. Ruggieri, P. Trahanias, M. Wilson. A standard reference model for intelligent multimedia presentation systems. Computer Standards and Interfaces, Vol. 18, pp. 477-496, Elsevier Science, 1997.
[Boyle & Encarnacion]
C. Boyle and A.O. Encarnacion. MetaDoc: an adaptive hypertext reading system. User Modeling and User-Adapted Interaction, Vol. 4, pp. 1-19, Kluwer academic publishers, 1994.
[Brusilovsky, 1996]
P. Brusilovski. Methods and Techniques of Adaptive Hypermedia. User Modeling and User-Adapted Interaction, Vol. 4, pp. 21-45, Kluwer academic publishers, 1994.
[Brusilovsky et al., 1998]
P. Brusilovsky, J. Eklund., E. Schwarz. Web-based education for all: A tool for developing adaptive courseware. Computer Networks and ISDN Systems (Proceedings of Seventh International World Wide Web Conference), Vol. 30 (1-7), pp. 291-300, 1998.
[De Bra & Calvi, 1998]
P. De Bra and L. Calvi. Towards a Generic Adaptive Hypermedia System. Proceedings of the Second Workshop on Adaptive Hypertext and Hypermedia, pp. 5-11, 1998.
[De Bra et al., 1999]
P. De Bra, G.J. Houben, H. Wu. AHAM: A Dexter-based Reference Model for Adaptive Hypermedia. Proceeding of the ACM Conference on Hypertext and Hypermedia, pp. 147-156, 1999.
[Halasz & Schwartz, 1994]
F. Halasz and M. Schwartz. The Dexter Hypertext Reference Model. Communications of the ACM, Vol. 37, nr. 2, pp. 30-39, 1994.
[Hothi & Hall, 1998]
J. Hothi and W. Hall. An Evaluation of Adapted Hypermedia Techniques Using Static User Modeling. Proceedings of the Second Workshop on Adaptive Hypertext and Hypermedia, pp. 45-50, 1998.
[Pilar da Silva et al., 1998]
D. Pilar da Silva, R. Van Durm, E. Duval, H. Olivié. Concepts and documents for adaptive educational hypermedia: a model and a prototype. Proceedings of the Second Workshop on Adaptive Hypertext and Hypermedia, pp. 35-43, 1998.

Paul De Bra is also affiliated with the University of Antwerp and the "Centrum voor Wiskunde en Informatica" in Amsterdam.