Defining Usability Laura Leventhal and Julie Barnes Computer Science Dept.
Sources • Chapter 3, Protobook
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.
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.
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
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
Social Accceptability Utility Easy to Learn Efficient to Use System Acceptability Usefulness Usability Easy to Remember Few Errors Cost Practical Acceptability Subjectively Pleasing Compatibility Reliability Nielson, 1993
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.
*Independent Variables System Functions Task Match Ease of Use Ease of Learning User Characteristics Knowledge Discretion Motivation Task Characteristics Frequency Openness *Dependent Variables User Reaction Implicit Cost/Benefit Analysis Positive Outcome Negative Outcome Restricted Use Non-Use Partial Use Distant Use Eason, 1984 (2)
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.
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.
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.
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.
Openness X Frequency X Task Characteristic Shackel Nielsen Eason User Characteristic Shackel Nielsen Eason Knowledge X Motivation X Discretion X System Characteristic Shackel 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” Pleasing to users called “attitude” called “subjectively pleasing” Adaptable called “flexibility “ Match between system function and task called “task match” Summary of Models
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.
Applying (and understanding) Eason’s Model • Eason’s 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
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.
How to use the Eason Model • We need to operationalize the concepts that are included as both independent variables. • From the Kaplans - next slide
Causal Models User Reaction (Usability) Eason hypothesizes… Task Characteristics User Characteristics System (UI) Characteristics You translate abstract concepts into operational definitions You translate abstract concepts into operational definitions Measurementof frequency, openness Measurementof user reaction You demonstrate causality … Measurement of knowledge, discretion, motivation Measurement of ease of learning, ease of use, task match You conclude that the original abstract relationship was valid …
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 can’t 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.
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.
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 style, etc. Performance on a standard task Operational Definitions - Contextual Dimension of User Characteristics
Characteristic Frequency Openness Other Task Characteristics (not explicitly listed in Eason) Operational Definitions Counts in standard situation Expert evaluation Importance as scored by users or experts Dangerousness or safety critical aspects as scored by experts Operational Definitions - Contextual Dimension of Task Characteristics
Some Hints as to How Context Can Interact with User Interface Characteristics to Influence Usability: Task Characteristics • Frequency -- • If routine, frequently done task • want speed efficiency • If infrequently • want guidance • Openness -- • Structured, finite tasks like spell checking may be encoded so that user simply selects task, not steps • Open-ended tasks (drawing, word-processing) may need to allow user to define their own steps and inputs.
Some Hints as to How Context Can Interact with User Interface Characteristics to Influence Usability: User Characteristics • User Knowledge -- • If novices, need frequent reminders of next steps from the user interface • If expert, do not need frequent reminders but need speed and flexibility from the user interface
Operational Definitions - System Characteristics • System functions: • Task matching - will SW do the job? User ratings may be used here. • Ease of use - time or number of errors to complete a task, once learned • Ease of learning - time or number of errors to learn to use an interface • Note. “Task” is likely standardized
Typical Measures for Elements of Eason Model • User's self-assessment on a scale (user reaction) • User's years of training or number of courses taken (user knowledge) • User's score on a comprehension test; test is given after exposure to the system (ease of learning) • Number of errors on a standard task (ease of learning, ease of use) • Time to complete a standard task (ease of learning, ease of use) • Number of standard tasks completed in a set time period (ease of use) • Number of features used (user discretion) • Willingness of user to engage in task (user motivation) • Number of times a task occurs in a standard time period (frequency) • Number of variations on task, as specified by experts (openness)
Usability of What? • What part of the user 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 focus more on the specific user interface.
Conclusions - Chapter 3 • A dictionary definition of “usability” is not detailed enough to be useful. Instead we use models of usability. We can apply these models during development or to evaluate existing user interfaces. There are several popular models. • We discuss three models of usability, including models due to Shackel, Nielsen and Eason. • Shackel emphasizes measurement of a number of human factors, relating to human performance and attitude. • Nielsen emphasizes usability as part of a larger set of system characteristics. • Eason sees usability as “multi-variate”. It is the result of several interacting variables, including variables of the user interface and the context. • All three models emphasize • Ease of learning • Ease/efficiency of use • All three models suggest defining and measuring usability and the characteristics which determine usability.