In usability research it is a common practice to take young and healthy university students as participants for usability evaluations. This chapter focuses on the “weaker” mobile phone users, which have been mostly disregarded in this field: Learning-disabled children. Their interaction with mobile phones is compared to that of average children and students. Results show that the consideration of the “ergonomic worst case,” which means a user group with cognitive deficiencies, leads to qualitatively and quantitatively different insights into the impact of specific design decisions. In contrast, when only students are involved as participants in the evaluation of technical devices, the impact of characteristics of the user interface on the ease of use is dramatically underestimated. One factor hampering the ability of learning-disabled children to interact meaningfully with a technical device may be their big difficulty building a correct mental representation of it. Therefore, this process should be especially supported.
Key Terms in this Chapter
Learning-Disabled Children: Children with a developmental speed that is slower than that of average children (about 1 to 2 years behind). Usually learning-disabled also do not reach the highest stadium of cognitive development which is characterized by abstract reasoning about problems. Learning-disabled show a permanently constricted learning field, which means they are only susceptive to concrete and needs-related material, they have a reduced ability for abstractions, limited capacity to structure tasks and are generally slow, shallow and time-limited in their learning process.
Ecological Validity: Degree to which results of experiments are transferable to behavior in real world situations. The higher the ecological validity of an experiment the higher is the probability that results found in the experiment can be found in the same fashion in the field.
Ineffective Keystrokes: Measure for performance evaluation. Counting each key stroke carried out by a user that does not lead to any task related effect on the display enables to measure the difficulty imposed by the navigation keys of a mobile phone independent of the difficulties caused by the menu. Ineffective keystrokes include Hash (#) and asterisk (*) at any point within the menu, number keys when not task related as well as soft keys, function keys and scroll-buttons when not exerting a function.
Navigation Keys: Keys used to operate the menu of a mobile phone, usually consisting at least of two keys fro scrolling up and down within one level, one key for selection and one key for returning to higher menu levels.
Mobile Phone Menu: Form of displaying mobile phone functions that go beyond effectuation of calls to the user. Mobile phone menus usually have a hierarchical tree structure, which the user needs to navigate through via keys in order to find and select the desired function.
Mental Models: concepts in the mind of users about the functioning of devices, metaphors, and ideas which lead the user while interacting with the device.