1. What Do We Mean by Usability?

2. Defining Usability We’ll examine 3 models:
Nielsen, 1993
Shackel, 1986
Eason, 1984
Look for similarities and differences
Conclude with a definition, not a model
Nielsen, Jakob: Usability Engineering ©1993 by Academic Press
Shackel, B. (1986)
Eason

3. Nielsen, 1993 Figure 1 (p. 25) & section 2.2 (p.26)
Usability is only part of the whole issue of system acceptability
Users must be satisfied with the systems they use
Usability is a narrow concern, compared to the larger issue of system acceptability — whether the system is good enough to satisfy all of the needs and requirements of the users and other stakeholders (`user's’ clients and managers).
System acceptability = social acceptability + practical acceptability + usability.

4. Nielsen, (Figure)
Figure 1: A model of the attributes of system acceptability
From Usability Engineering
By Jakob Nielsen
Published by Morgan-Kaufmann ©1993
QA76.9.U83N

5. Shackel, 1986 Began development in 1959
An operational definition of usability
Effectiveness
Learnability
Flexibility
Attitude
Shackel, 1986
Emphasis of Shackel is on operationability. Can the definition be used?
Note some similarities to Nielson.

6. Eason, 1984 Widely-accepted model of usability.
Usability is influenced by a number of factors that interact with one another.
The major indicator of usability is whether the system or facility is used.

7. Eason, 1984 (Figure) Figure 4: A causal framework of usability.
K. D. Eason
Towards the experimental study of usability.
Behaviour and Information Technology v. 3 no. 2, pp. 133–143 (1984)
Figure from p. 138.

8. The Three Usability Models
No single definition
Nielsen emphasizes usability as part of larger system characteristics
Eason sees usability as the result of several interacting variables
Nielsen model  additive
Eason model  causal
All emphasize ease of learning & ease of use
Let’s look more closely at Eason’s model

9. Eason, 1984 (Task) Task characteristics Frequency Openness
Number of times task is performed by a user
Openness
Extent to which task is modifiable

10. Eason, 1984 (System) System characteristics Ease of learning
Effort required to understand & operate unfamiliar system
Ease of use
Effort required to operate system after it’s understood and mastered
Task match
Extent to which information and functions provided match user’s need
Ease of learning vs. ease of use
these are different
for example, a system that is easy to learn, via a dialog that guides the user through the tasks may be difficult to use because the dialog is actually in the way. Or a system which is difficult to learn initially (e.g. Unix) may be easy to use once the user has memorized the commands.

11. Eason, 1984 (User) User characteristics Knowledge Motivation
That user applies to task
May be appropriate or inappropriate
Motivation
Highly motivated users may expend more effort to overcome difficulties (problems, misunderstandings)
Discretion
User's ability to choose not to use some part of a system

12. Usability of What What part of the interface are we discussing?
Often think of it as applying just to user software (UI) but really should be applied to all facets of a product: manuals, installation, on-line help, training, hot-line, maintenance, etc.
These areas are receiving more attention. However, we will focus more on UI.
Norman’s idea (in POET and Norman & Draper) about system image

13. What to Do With Usability?
Once we have a definition of usability, we would like to do something:
Demonstrate or evaluate existing systems
Develop systems with a goal of usability

14. What Factors Relate to an Impression of Usability?
Look at Eason's model
User/system/task characteristics
Are main categories
Independent variables (IV)
User reaction
Dependent variables (DV)

15. Quick Definitions of IV and DV
Independent var.: Characteristic that you manipulate.
Its level or setting is independent of other variables.
Dependent var.: What you measure as a result.
Its value is dependent on your manipulations.
Figure shows a few of the many IVs for each.

16. How to Measure Usability?
Operational definition of usability
Statement of the operations that are necessary to:
Produce and
Measure the concept
A. Some concepts are easier to operationalize than others: e.g.. "errors" could be number of errors
B. Some concepts are difficult to operationalize: "motivation", "task match".

17. Here Are Some Operational Definitions
User characteristics:
Knowledge — expert/novice, age
Motivation
Cognitive aspects — memory and memorization performance errors
Problem solving style
Learning style, etc.

18. More Operational Definitions
System functions:
Task matching — will S/W do the job?
Ease of use
Ease of learning
Transparency

19. More Operational Definitions
Task characteristics:
Frequency
If routine, frequently done task
Want speed efficiency
If infrequently
Want guidance
Openness
Structured, finite tasks, e.g. spell checker
Open-ended tasks, e.g. drawing, word-processing

20. The ISO Definition ISO-DIS 9241-11 says: 3 Parts:
User
Task
Context
3 Measures:
Accuracy
Speed
Satisfaction
ISO-DIS says:
The effectiveness, efficiency, and satisfaction with which specified users achieve specified goals in particular environments.
Three things:
User
Task
Context
We can’t control the context but we can specify the other two
Three measures:
Affective (happiness, satisfaction)
Effectiveness (accuracy, correctness)
Efficiency (speed)

21. Typical Usability Measures
Nielsen (pp. 194 – 195)
All quantifiable
Not all objective
Nielsen, J: Usability Engineering, © 1993 by Academic Press