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216 Selected Studies on Software and Information Systems
The system based on overlay model thus provides “real” personalization based on
characteristics of each individual user.
The advantage of stereotype model is its ease of use and relatively simple initialization. User
can be assigned a stereotype according to answers of few questions or eventually according
to the initial behavior in the system (e.g., based on a rule: “user with a stereotype X will
perform Y as a first action”). Stereotypes can be organized hierarchically and initially
assigned stereotype can be thus further refined as new information about user become
available.
Disadvantage of a stereotype model lies in limited personalization possibilities if the system
does not concern the user as an individual and stereotypes are rather coarse grained (which is
often the case if we define stereotypes manually). This results in “one-size-fits-all” problem
of personalization for all users within one stereotype.
An overlay model deals with this kind of a problem. It creates a separate user model for
each user by creating additional layer with user characteristics above the domain model.
A problem of overlay model lies in increased demand for system resources as the number
of user increase. It is necessary to use a specialized approach in case of a large or even
open information spaces. The solution could be to create an overlay only in such parts of
information space, which are related to the particular user (which were already visited by the
user). Another solution is to create a more loose coupling of user and domain model, where
characteristics relate to types and/or attributes of concepts, not the concepts themselves. For
example, in the domain of job offers, user model does not hold characteristics related to each
and every job offer, but rather characteristics related to attributes of a job offer, which are
certainly shared among several job offers.
User model used in adaptive web-based systems can be represented in multiple ways [2].
Representations differ in level of expressivity and flexibility they provide as well as in shareability
between several adaptive applications or possibilities of further work with characteristics.
8.1.1 Vector
The simplest way to represent user’s characteristic is to represents user’s attitudes to all
concepts of used domain in multiple vectors, each representing particular information about
a user (whether the user visited a concept, understood the concept, liked the concept etc.).
However, this solution is applicable only on closed and non-changing information spaces.
Such user model representation is easy to implement and work with, but lacks direct shareability
feature.
8.1.2 Relational database
Many systems use relation databases [12] to represent user models. Since most of the
domain-related information of web-based systems is already stored in a relational database,
it is straightforward and easy to add user-related information as well. Compared to the vector
representation, it allows us to store attribute-value pairs which are not directly connected to
used domain.
Even if use of relational databases usually does not require additional resources and
brings several advantages (performance, security, overall mature of used technologies), it