## 2 Analysis of the State of the Art
### [PERSON] _et al._
#### 2.1.2 Users Models Comparison  
In this section, we emphasize our user modeling perspective. There are several points of view regarding the user modeling requirements. Here the HCI perspective is taken. This means, (as [PERSON] states in [18]) that the user model refers to user characteristics using a certain system (see Section 2.2.1.). In the AI perspective the works by [PERSON] and [PERSON] are significant. The authors design generic user model systems (known as shell systems) under this viewpoint [17; 37]. Now that the HCI viewpoint has been remarked, we emphasize the amount of different domains that are addressed in the literature considering user modeling. From product design to multimedia and user interfaces adaptation, every approach follows the same purpose: to consider several user characteristics to improve the system and user's satisfaction and product or service usability. However, although these solutions share the same objectives, the considered characteristics differ a lot. For ubiquitous and more context-aware domains, activities are taken into account. For example, [PERSON] [21]_et al_. discuss about an ontology based architecture, which aims to be generic through collecting user data by two different ways (explicitly and implicitly). [PERSON] [29]_et al_. also take an ontological point of view to avoid the problem of domain dependency (among others) by designing a more general, comprehensive and extensible ontology. [PERSON] _et al_. [20] remark in the presented ontology the importance of time. Regarding the studied domain (web browsing) time is significant because it might help characterizing the user considering the spent time reading an article or visiting a website. Well known and popular recommendation systems, as Youtube, utilize this information combined with different explicit and implicit sources from the user interaction to make proper recommendations [38]. User activities have also been considered as relevant for many authors in the literature. The first example is the Doppelganger system [39] (1991), which uses activities for sharing relevant user information to different applications. In the same way, [PERSON] _et al_. [27], [PERSON] _et al_. [30] and [PERSON] _et al_. [36] modelled activities to take user's behavior and interaction into account for the proposed classifications and systems. For [PERSON] and [PERSON] [22] activities are also vital within context-aware environments. As occurs with context modeling (see Section 2.2.2.) many different techniques are available for the model representation. This usually depends either on the developer, because of his/her experience, or in the system's technical characteristics. For example, if the system is able to performs inference with the user data an ontology based representation could be more helpful than an object based one. [PERSON] and [PERSON] [40] demonstrate that ontological modelling is more appropriate for ubiquitous computing environments. It is also common to model medical related characteristics of the user. For example, the works by [PERSON] _et al_. [5] and [PERSON] _et al_. [27] consider physical, cognitive an sensory capabilities. [PERSON] _et al_. [36] also model several user abilities for performing different tasks and activities. The problem is that being aware of these capabilities is difficult and, in some cases, poorly practical. For example, measuring the sight capability of one individual requires medical experience or advise. Besides, people with the same affection do not respond in the same way. A person who was born blind would interact differently with the environment than another who has been losing sight during his/her life. The precise same disability, e.g., tunnel vision, might affect different people in many different ways depending on their personal skills, e.g., adaptability, orientation, etc. This leaves an open issue which leads us to an idea. What if, instead of modeling medical skills (disabilities), we were able to model user's capabilities?