## 1 Introduction
### Motivation: Human-Computer Interaction and Users' Context Disabilities  
**Definition 1**. : _Designing an object to be simple and clear takes at least twice as long as the usual way. It requires concentration at the outset on how a clear and simple system would work, followed by the steps required to make it come out that way--steps which are often much harder and more complex than the ordinary ones. It also requires relentless pursuit of that simplicity even when obstacles appear which would seem to stand in the way of that simplicity [6]._  
This cite by [PERSON] [6] in 1977 already pointed out the problems that designing a product entails. One of the most significant issues to face during this process is the usability. According to the ISO/IEC 9126 standard, quality represents a property of the software product defined in terms of a set of interdependent attributes, i.e., usability, security, reliability, performance, complexity, readability and re-usability expressed at different levels of detail and also taken into account the particular context of software use [7,8]. At this point, the ISO 9241-11 standard [9] states that usability is the extent to which a product can be used by specified users to achieve specified goals with effectiveness, efficiency and satisfaction in a specified context of use. The interaction with devices needs to be satisfactory for the users. The ISO/IEC 9126-1 [7] presents and details a two-part model for software product quality:  
1. Internal and external quality (see Figure 1): Internal Quality is the totality of attributes of the software product from an internal view, e.g., spent resource. It is measured and improved during the code implementation, reviewing and testing. External Quality is the quality when software is running in terms of its behavior, e.g., number of wrong expected reactions. It is measured and evaluated for software testing in a simulated environment [8]. 2. Quality in use (see Figure 2): It is the capability of the software product to enable specified users to achieve specified goals with effectiveness, productivity, safety and satisfaction in specified contexts of use. Figure 1: Quality model for external and internal quality [7]. However, the design process becomes troublesome because of the nature of each user. Users are very different from each other. They like different things and they sense and perceive in different ways. Moreover, they have different capabilities. Besides, there are several groups which suffer these differences more deeply: people with disabilities and the elderly. The first group suffers from different disabilities which are responsible for limiting several capabilities in a certain way. For example, users with sight disabilities will suffer from interaction problems with their devices if this interaction is based on visual stimulus, e.g., using a device display. On the other hand, elderly people usually suffer from similar interaction troubles due to their aging. As their senses tend to tire their capabilities and interaction levels decrease [5]. Current technology trends try to reduce the interaction barriers that elderly suffer with current devices. Mobile phones have audio control interaction and screen magnification, TVs have zoom and subtitles capabilities, _etc._ Nevertheless, the elderly are used to the products they already know [10, 11]. For these groups the designed devices should be:  
* _Easy to use_, so the users are able to use them to their own purposes. * _\"Easy to learn\"_. This way, the final purpose of the device should be affordable in an acceptable time interval. * _Easy to recall_, so the users are able to remember how to interact with the device. Nonetheless, we are not exempt of suffering from similar situations. There are many conditions which cause people without disabilities to feel like they have one. Making a phone call in a very sunny day or send an email when it is raining are examples of several context situations which limit users' capabilities.