User models are used to generate or adapt user interfaces at runtime, to address particular user needs and preferences. User models are also known as user profiles, personas or archetypes. They can be used by designers and developers for personalisation purposes and to increase the usability and accessibility of products and services.
User Model, Standardization, Interoperability, User Preferences, Adaptation, User Profile, Personalization.
User models are explicit representations of the properties of an individual user including needs, preferences as well as physical, cognitive and behavioural characteristics. The characteristics are represented by variables. The user model is established by the declaration o these variables. Due to the wide range of applications, it is often difficult to have a common format. A user model can be seen from a functional/procedural point of view or from a more declarative point of view. In the first case the focus is laid on processes and actions that the user carries out to accomplish tasks. In the second case the focus is set on definitions and descriptions of the properties and charecterstics of the user.
A user profile is an instantiation of a user model representing either a specific real user or a representative of a group of real users.
Beside the user model there are other related models e.g. task model, device model or environment model. Task models describe how to perform activities to reach users' goals (see W3C Model-based User Interfaces Incubator Group). A device model is a representation of the features and capabilities of one or several physical components involved in user interaction. The device model expresses capabilities of the device. An environment model is a set of characteristics used to describe the use environment. It includes all required contextual characteristics.
User models can be used to reason about the needs, preferences or future behavior of that user. One of the main problems is to integrate user profiles (an instantiation of a user model ) supporting different user models within a service/application, or migrating profiles from one service/application to another. This can be attributed to the broad variety of user profiles and the incompatibilities that can occur among them. For example, differences in user profiles can occur due to differences in scope of the modeling, source of information for the modeling, and time sensitivity of the model and update methods (static vs. dynamic model). Other factors that further increase the variety of models include:
- Physical (mechanics and control) and cognitive processes that underlie the users’ actions;
- Differences between the user's skills and expert skills, or the user's behavioral patterns and preferences;
- Whether the models apply to individual users or entire groups of users such as “communities”.
User modeling covers many different aspects, including the interaction of the user with interfaces and devices, the analysis of user tasks and the analysis of user characteristics (sensory, physical and cognitive abilities, psychological and behavioral characteristics). It is necessary to research similarities among different modeling approaches and identify ways for making user profiles portable. A common standard for interoperability could be an approach to achieve this goal. Application and nature of user models are very heterogeneous among different contexts, and this has to be reflected in the standardization activities. The goal is therefore not to create a new common user model standard, but to make user models interoperable through standardization of common aspects and creation of interfaces between them. This will support the exchange and sharing of user profiles among different services and contexts.
There are a plethora of systems developed in human computer interaction during the last three decades that use different types of user models . Many of them modeled users for certain applications - most notably for online recommendation and e-learning systems. There is a bunch of application-independent models which merges psychology and artificial intelligence to model human behavior in detail. In theory they are capable of modeling any behavior of users while interacting with environment or a system. This type of models is termed as cognitive architecture and has also been used to simulate human machine interaction to both explain and predict interaction behavior. A simplified view of one of these cognitive architectures is known as the GOMS model  and still now is most widely used in human computer interaction though it does not consider people with disabilities or non-expert users in detail.
The emergence of mobile devices showed the need for a description of device characteristics so content providers could customize their offers to capabilities of devices. The World Wide Web Consortium (W3C) developed the Composite Capabilities/ Preference Profiles (CC/PP) framework . CC/PP offers the possibility to define user and device profiles for an adequate adaptation of content and presentation for internet services. CC/PP is based upon  a general-purpose metadata description language. RDF provides the framework with the basic tools for both vocabulary extensibility, via XML namespaces, and interoperability. RDF can be used to represent entities, concepts and relationships in the web. So far, the only practical implementation of CC/PP is the user agent profile (UAProf), developed by the Open Mobility Alliance (formerly the WAP-Forum) and targeted to mobile devices. This approach was extended by further developments by W3C groups in the W3C Delivery Context Ontology; it provides a formal model of the characteristics of the environment in which devices interact with the web or other services. The delivery context includes the characteristics of the device, the software used to access the service and the network providing the connection among others. The universal remote Console - URC Standard (ISO/IEC 24752) the goal of URC technology is to allow any device or service to be accessed and manipulated by any controller. Users can then select a user interface that fits their needs and preferences, using input and output modalities, and interaction mechanisms that they are familiar with and work well with them.
With the explosion of the web, and e-commerce in particular, several commercial user modeling tools appeared in the market with the objective of adapting content to users’ preferences. Standards and recommendations in this area had to cope with the spread of service-oriented architectures in ubiquitous environments and to cover workflow and user interface aspects e.g. UsiXML , EMMA (Extensible Multi Modal Annotation mark-up language) and MARIA XML in all these frameworks contains a user model component but does not cover all user modeling aspects. Another major source for the development of user models was the E-Learning sector, here we mention IMS AccLIP (Access For All Personal Needs and Preferences Description for Digital Delivery Information Model) and AccMD, which have been internationalized in the ISO/IEC JTC1 Individualized Adaptability and Accessibility for Learning, Education and specification for the User Modeling software Training (ISO/IEC 24751-1:2008).
There is additional need in user models for awareness of context, location and emotional state, as well as seamless roaming and portability , the key issue here is as well the need to cope with demands of different granularity requirements as in applications at design time (simulation), where detailed information are required and runtime adaptations, where less detailed information are sufficient.
All these developments show the need for developing interoperability standards that can respond not only to everyday scenarios nowadays, but also can be flexible enough to cope with future scenarios coming from ubiquitous, wearable computing and Ambient Assisted Living.
Considering all the variety of approaches for user modeling, it becomes challenging to define what a user model actually is. This lack of definition also makes the interoperability of user models difficult. On the other hand, there is too many standards about human factors, user interface design, interface description language, workplace ergonomics, and so on, that can be used to develop user models. In this context the aims should be to develop:
- A standard interoperability model providing API's for differnt purposes and applications
- Common data storage format for user profiles
- Common calibration / validation technique
- Collaboration on ethical issues
- Ensuring sustainability by making them available within a standard
- Mechanisms for exchanging user profile data between sources
- Protection mechanisms for privacy issues
- Control mechanisms for user profile data exposure
This work is being successively worked on by many organisations, initiatives and projects e.g. the VUMS cluster, which initiated its work by defining a common glossary of terms  which enable user model developers to exchange concepts. Later it proceeded to define a set of variables to describe a user .
- VUMS: Cluster on Virtual User Modelling and Simulation
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- VUMS white paper
- VUMS Terms Definition
- VUMS user model variables
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