1 Defining Usability Laura Leventhal and Julie Barnes Computer Science Dept.
2 Sources Chapter 3, Protobook
3 Definition of Usability Usability would seem easy enough to define. We could look it up in a dictionary or technical manual. The international standard, ISO 9241-11 defines usability as: Usability: 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.
4 Is the Definition Enough? Note that this definition is still not detailed enough to evaluate whether a system is usable or not, although it certainly indicates what some characteristics of a usable system might be. How could we extend a definition into something that could actually be used to evaluate usability? Many authors have defined models of usability. A model not only states the characteristics of a usable interface but also indicates how those characteristics fit together and what they mean.
5 Models of Usability There are at least three models. We will look at each model. Look for similarities and differences. Shackel, 1986 Nielsen, 1993 Eason, 1984
6 Shackel, 1986 Effectiveness Better than some required level of performance By some required percentage of the specified target range of users Within some required proportion of the range of usage environments Learnability Within some specified time from installation and start of user training Based on some specified amount of training and user support Within some specified re-learning time each time for intermittent users Flexibility Allowing adaptation to some specified percentage variation in tasks and/or environments beyond those first specified Attitude Within acceptable levels of human cost in terms of tiredness, discomfort, frustration and personal effort
7 Nielson, 1993 System Acceptability UsabilityUsefulness Practical Acceptability Cost Compatibility Reliability Easy to Learn Efficient to Use Easy to Remember Few Errors Subjectively Pleasing Social Accceptability Utility
8 Eason, 1984 Most 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. The Eason model is emergent and was based on field studies that Eason and his team made of real projects.
9 Eason, 1984 (2) System Functions Task Match Ease of Use Ease of Learning User Characteristics Knowledge Discretion Motivation Task Characteristics Frequency Openness User Reaction Implicit Cost/Benefit Analysis *Independent Variables *Dependent Variables Positive Outcome Negative Outcome Restricted Use Non-Use Partial Use Distant Use
10 Eason, 1984 (Task) Task means what you do with the user interface. Task characteristics frequency number of times task is performed by a user. openness extent to which task is modifiable. For example, in a word processor interface, spell checking is a closed task, while writing a poem is open Note the characteristics of the task is independent of the platform in which the task is being performed.
11 Eason, 1984 (User) Eason recognized that characteristics that the user brought to the task and the user interface would influence their experience. User characteristics - knowledge the knowledge that the user applies to the task. The knowledge may be appropriate or inappropriate. motivation If the user has a high degree of motivation, then more effort will be expended in overcoming problems and misunderstandings. discretion user's ability to choose not to use some part of a system.
12 Eason, 1984 (System) System in Eason model refers to the user interface System characteristics - ease of learning effort required to understand and operate an unfamiliar system. ease of use effort that is required to operate a system once it has been understood and mastered by the user. task match extent to which information and functions that a system provides matches the needs of the user.
13 Conclusions - Usability Models No one definition or model of usability. Structure of the Models 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 Context of the models Nielsen - user interface usability in the context of a software engineering project Eason - usability in the context of the environment in which the user interface will be used. Similarities All three models emphasize ease of learning and ease of use.
14 Summary of Models System CharacteristicShackel Nielsen Eason Ease to learn initially called learnability called easy to learn called easy to learn Ease of relearning for intermittent users called learnability called easy to remember Matches target performance level called effectiveness called efficient to use called easy to use Low error rate and recoverability called errors Task Characteristic Shackel Nielsen Eason Pleasing to users called attitude called subjectively pleasing Openness X Frequency X Adaptable called flexibility Match between system function and task called task match User Characteristic Shackel Nielsen Eason Knowledge X Motivation X Discretion X
15 Who Cares - What to Do With Usability Models? 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 We can use a usability model for either of these goals. For the remaining discussion we will discuss the Eason model in more detail and show how you might use it. This detail should suggest to you some ways that the Eason model could be used for evaluation or design.
16 Applying (and understanding) Easons Model Easons model is saying that usage context, in combination with user interface characteristics drive and determine usability. Eason's model has two parts The input to his model is User/system/task characteristics are main categories independent variables (IV) The outcome of his model is User Reaction - dependent variable - DVs
17 Causal Models and Quick definitions of IV and DV A causal model is one that makes predictions about causality. In a causal model, if you manipulate the inputs, the outcome is a result. Elements of a causal model Independent variable: a characteristic that you manipulate. It's level or setting is independent of any other variables. Dependent variable: what you measure as a result. Its value is dependent on your manipulations. The Eason model is a causal model. The independent variables that he has identified with his contextual dimensions are just a few of the variables that he could have selected.
18 How to use the Eason Model We need to operationalize the concepts that are included as both independent variables. From the Kaplans - next slide
19 Causal Models Eason hypothesizes Task Characteristics User Reaction (Usability) User Characteristics System (UI) Characteristics Measurementof frequency, openness You translate abstract concepts into operational definitions Measurement of ease of learning, ease of use, task match Measurement of knowledge, discretion, motivation Measurementof user reaction You demonstrate causality You translate abstract concepts into operational definitions You conclude that the original abstract relationship was valid
20 What Does this Mean for Us? We are not testing the Eason model experimentally. Rather we are accepting it. We accept that the contextual variables of task and user and the system (user interface) characteristics influence usability. Generally we cant change the context (contextual variables) for a project. But we can define and measure them and use the information in design. By good usability engineering, we can influence system (user interface) characteristics, in the given context, and therefore improve usability. In order to determine if our user interface supports system characteristics like ease of use, we need to identify ways to define and measure these characteristics. Ultimately we need to measure user reaction (usability) as well to verify that our system, in its context, we need to define and measure this concept as well.
21 How to Assess and Measure Usability? Eason claims that usability is reflected by user reaction. So the Usability Engineer needs an operational definition of user reaction. This definition would be a statement of the operations that are necessary to produce and measure the concept. For example, user reaction might be operationalized as responses to a survey.
22 Operational Definitions - Contextual Dimension of User Characteristics Characteristic Knowledge Motivation Discretion - Other User Characteristics (not explicitly listed in Eason) Operational Definitions Expert/Novice Categories Age of User (older users may have less knowledge) In trying situation, score users willingness to continue Measurement of users ability to discriminate between similar situations. memory and memorization performance errors problem solving style learning s