1. Human Computer Interaction
Week 9
Usability Concept

2. A Model of Computer Usage
Norman (1987): The perfect interface environment is one in which the interface is transparent to users, with nothing between them and their tasks

3. A Model of Computer Usage
Norman (1986) states that the major issues which must be addressed in order to improve the usability of computer systems are represented by two gulfs: - the Gulf of Evaluation - the Gulf of Execution

4. User Centered Design (Norman 1987)
The conceptual model, which is the designers’ view of the system.
The system image, which is the image the system presents to the user.
The mental model, which is the model the user develops of the system, and which is mediated to a great extent by the system image.

5. Usability Definition Miller (1971): ease of use
Shackel (1981): effectiveness in human performance
Brooke (1990): user, task, condition
Shackel (1991): operational criteria - Effectiveness - Learnability - Flexibility - Attitude
Jordan (1995): learning process - Guessability - Learnability - Experienced User Performance (EUP) - System Potential - Reusability

6. Usability Definition (Shackel 1981)
the capability in human functional terms to be used easily and effectively by the specified range of users, given specified training and user support, to fulfil the specified range of tasks, within the specified range of environmental scenarios

7. Definition by Brooke (1990)
the users who will use the product
the tasks for which the product will be employed
the conditions under which those users will perform those tasks

8. Usability Definition (Shackel 1991)
Effectiveness
the required range of tasks must be accomplished at better than some required level of performance. (e.g., in terms of speed and errors)
by some required percentage of the specified target range of users.
within some required proportion of the range of usage environments.

9. Usability Definition (Shackel 1991)
Learnability
within some specified time from commissioning and start of user training.
based upon some specified amount of training and user support.
and within some specified relearning time each time for intermittent users.

10. Usability Definition (Shackel 1991)
Flexibility
with flexibility allowing adaptation to some specified percentage variation in tasks and/or environments beyond those first specified.

11. Usability Definition (Shackel 1991)
Attitude
and within acceptable levels of human cost in terms of tiredness, discomfort, frustration and personal effort.
so that satisfaction causes continued and enhanced usage of the system.

12. Usability Definition (Jordan 1995)
Guessability. First Time Use
Learnability. Learning process to achieve a competent level of performance
Experienced-User Performance (EUP). Experienced users can achieve specified tasks with a particular interface.
System potential. The optimum level of system performance to complete specified tasks with an interface.
Re-usability. Users can achieve specified tasks with a particular interface after a comparatively long time away from these tasks.

13. Principles to support usability
Learnability the ease with which new users can begin effective interaction and achieve maximal performance
Flexibility the multiplicity of ways the user and system exchange information
Robustness the level of support provided to users in determining successful achievement and assessment of goals

14. Learnability
Predictability: determine effect of action based on past interaction history
Synthesizability: assess the effect of past operations on the current state
Familiarity: user’s knowledge / experience can be applied when interacting with a new system
Generalizability: extend knowledge of specific interaction to other similar situations
Consistency: likeliness in input-output behavior from similar situations

15. Flexibility
Dialog initiative: allow the user freedom from artificial constrains on input dialog
Multi-threading: system ability to perform multi-task at a time
Task Migratability: pass control of a task to user or system of shared between them
Substitutivity: allow equivalent values of input/output to be substituted for each other
Customizability: modifiability of the user interface by the user or the system

16. Robustness
Observability: user ability to evaluate the internal state of the system from its perceivable representation
Recoverability: user ability to correct action once an error has been recognized
Responsiveness: how the user perceives the rate of communication with the system
Task Conformance: the degree to which the system services support all of the tasks the user wishes to perform

17. The criteria for direct manipulation (by Shneiderman)
Visibility of the objects of interest
Incremental action at the interface with rapid feedback on all actions
Reversibility of all actions, so that users are encouraged to explore without severe penalties
Syntactic correctness of all actions, so that every operation is a legal operation
Replacement of complex command languages with actions to manipulate directly the visible objects

18. Further Reading
Dix, chapter 4, pp