What Do We Mean by Usability?

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

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.

Nielsen, 1993 (Figure) Figure 1: A model of the attributes of system acceptability From Usability Engineering By Jakob Nielsen Published by Morgan-Kaufmann ©1993 QA76.9.U83N54 1993

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.

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.

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.

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

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

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.

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

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

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

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)

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.

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".

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.

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

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

The ISO Definition ISO-DIS 9241-11 says: 3 Parts: User Task Context 3 Measures: Accuracy Speed Satisfaction ISO-DIS 9241-11 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)

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