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by Whitney Quesenbery
The definition of usability is sometimes reduced to "easy to use," but this over-simplifies the problem and provides little guidance for the user interface designer. A more precise definition can be used to understand user requirements, formulate usability goals and decide on the best techniques for usability evaluations. An understanding of the five characteristics of usability effective, efficient, engaging, error tolerant, easy to learn helps guide the user-centered design tasks to the goal of usable products.
MEANINGS OF USABILITY
The word "usability" has become a catch-phrase for products that work better for their users, but it is difficult to pin down just what people mean by it. Is usability
These different meanings can be described in four key requirements:
DEFINING EASE OF USE
The definition of usability in the ISO 9241 standard is:
This definition can be expanded, and made more comprehensive, by including five characteristics which must be met for the users of a product:
Effectiveness is the completeness and accuracy with which users achieve specified goals. It is determined by looking at whether the users goals were met successfully and whether all work is correct.
It can sometimes be difficult to separate effectiveness from efficiency, but they are not the same. Efficiency is concerned primarily with how quickly a task can be completed, while effectiveness considers how well the work is done. Not all tasks require efficiency to be the first principle. For example, in interfaces to financial systems (such as banking machines), effective use of the system -- withdrawing the correct amount of money, selecting the right account, making a transfer correctly are more important than marginal gains in speed. This assumes, of course, that the designer has not created an annoying or over-controlling interface in the name of effectiveness.
The quality of the user assistance built into the interface can have a strong impact on effectiveness. The effectiveness of an interface often relies on the presentation of choices in a way that is clearly understandable to the user. The more informative an interface can be, the better users are able to work in it without problems. Good interface terminology will be in the users language and appropriate to the task.
Another design strategy to increase effectiveness is to offer redundant navigation, especially for ambiguous situations. Although this may create inefficient paths, it allows the user to work effectively by making more than one choice lead to the correct outcome. This can be especially valuable in interfaces which support infrequent users or those often unfamiliar with the content domain.
Efficiency can be described as the speed (with accuracy) in which users can complete the tasks for which they use the product. ISO 9241 defines efficiency as the total resources expended in a task. Efficiency metrics include the number of clicks or keystrokes required or the total time on task
It is important to be sure to define the task from the users point of view, rather than as a single, granular interaction. For example, a knowledge base which doled out small snippets of information might be very efficient if each retrieval was considered one task, but inefficient when the entire task of learning enough to answer a users question is considered.
Navigation design elements such as keyboard shortcuts, menus, links and other buttons all have an impact on efficiency. When they are well-designed, with clearly expressed actions, less time and effort are needed for the user to make navigation and action choices..
Making the right choices for efficient use of the software depends on an understanding of the users and how they prefer to work. For example, are they likely to use the interface infrequently or to be habitual users who might learn hidden controls and shortcuts? Do they use the keyboard, mouse or other input devices? For example, keyboard shortcuts can be extremely efficient for proficient users who work with the interface intensively. If they are the primary interaction tool, they can slow down users who are unfamiliar with them, or with the software. Similarly, an interface structured around a set of hierarchical choices which may be the best solution for one-time or infrequent users, might be frustratingly slow as the only way of interacting with a frequently-used program.
An interface is engaging if it is pleasant and satisfying to use. The visual design is the most obvious element of this characteristic. The style of the visual presentation, the number, functions and types of graphic images or colors (especially on web sites), and the use of any multimedia elements are all part of a users immediate reaction. But more subtle aspects of the interface also affect how engaging it is. The design and readability of the text can change a users relationship to the interface as can the way information is chunked for presentation. Equally important is the style of the interaction which might range from a game-like simulation to a simple menu-command system.
Like all usability characteristics, these qualities must be appropriate to the tasks, users and context. The style of engagement that is satisfying for a repetitive work tool is different than an e-commerce site. Even within the same class of interfaces, different users may have widely divergent needs. What is important is that the design meet the expectations and needs of the people who must use the interface.
The ultimate goal is a system which has no errors. But, product developers are human, and computer systems far from perfect, so errors may occur. An error tolerant program is designed to prevent errors caused by the users interaction, and to help the user in recovering from any errors that do occur.
Note that a highly usable interface might treat error messages as part of the interface, including not only a clear description of the problem, but also direct links to choices for a path to correct the problem. Errors might also occur because the designer did not predict the full range of ways that a user might interact with the program. For example, if a required element is missing simply presenting a way to fill in that data can make an error message look more like a wizard. If a choice is not made, it can be presented without any punitive language. (However, it is important to note that it is possible for an interface to become intrusive, or too actively predictive.)
For those errors which are out of the control of the interface system failures or other disasters - take a lesson from flight attendants and quietly, calmly guide the user through the process of helping the program recover from the problem.
Some guidelines for preventing errors are:
Easy to Learn
One of the biggest objections to "usability" comes from people who fear that it will be used to create products with a low barrier to entry, but which are not powerful enough for long, sustained use.
But learning goes on for the life of the use of a product. Users may require access to new functionality, expand their scope of work, explore new options or change their own workflow or process. These changes might be instigated by external changes in the environment, or might be the result of exploration within the interface.
An interface which is easy to learn allows users to build on their knowledge without deliberate effort. This goes beyond a general helpfulness to include built-in instruction for difficult or advanced tasks, access to just-in-time training elements, connections to domain knowledge bases which are critical to effective use.
Allow users to build on not only their prior knowledge of computer systems, but also any interaction patterns they have learned through use in a predictable way. Predictability is complementary to interface consistency. A consistent interface ensures that terminology does not change, that design elements and controls are placed in familiar locations and that similar functions behave similarly. Predictability expands this to place information or controls where the user expects it to be. This concept has been discussed in connection with Palm Pilot design and especially important if you make an interface which goes beyond the boundaries of simple platform design standards. Good use of predictability requires careful user analysis and observation, but can make new functions easy to learn by providing controls where the user expects them to be.
WORKING WITH THE FIVE E'S
Finding the right balance between the usability characteristics for the specific design context is an important part of the user analysis. The difference in emphasis is helpful in understanding distinctions between user groups and in thinking through the implications for the interface design. Two fictional examples show this at work.
A Corporate Human Resources (HR) Site
A typical web knowledge management system is used by employees to look up information about their benefits, including options for leave, medical benefits and scholarship support. These users might express the following needs (in order of importance)
A Conference Registration System
Contrast the previous example with users of an online conference registration system. These users (also fictional) will use this site once, but are spending a relatively large sum to register. Their experience of the conference itself may depend on the success of the registration system.
Thinking though users perspective
Although the examples above are fictional, they illustrate one way to use the five usability characteristics to understand the user requirements and mental model for a task. By breaking down the generalized concept of usability into specific areas, the users can be understood in a multi-dimensional way, and usability becomes more than a simple requirement that the program be "easy to use."
A useful exercise is to write a statement for each characteristic for each user group. These statements can be written in the third person (as above) or can be turned into first person statements as a way of capturing a sense of the emotion or tone surrounding each statement. Where direct quotes from users are available, they add richness and credibility. Sometimes the directness of the quote or the diversity of users that the quotes show can be helpful in making users come alive for both to both designers and developers.
There are several benefits of this exercise. The first is to help specify the user groups. When a group of statements seems correct for one user, but not for another, this may be exposing important differences in user requirements. Another is to force the user analyst into a clear and concise expression of user needs. Finally, it can be a useful tool to build a consensus within a team on the user analysis.
This exercise can be done at the beginning of a project, even before any user analysis or observation has been done. In this version, the work focuses on the groups current understanding of users. Points of disagreement indicate a need for better understanding of users. Points of agreement can be confirmed through analysis. The set of statements for each identified target user group serves as a benchmark for future work.
After user analysis, the exercise is repeated. Places where the teams initial version differs significantly from the post-analysis version need careful attention to be sure the implications for the design are understood.
Connection to usability goals
Usability goals can also be tied to the five characteristics. Each user need statement can be turned into a usability goal or requirements. For example, requirements can be specified with a range of acceptable values, such as:
One aspect of transforming archetypal user statements into usability goals must be stressed. Users often place a low importance on characteristics which they simply expect to be well represented in the interface. An example of this is the assumption by the conference registration system user that the task was simple enough that ease of learning was not a critical factor. In creating usability goals, the emphasis must be reversed, with a priority placed on meeting those base-line assumptions. An interface that fails in this will not be usable, even if it meets other requirements. In fact, this basic failure will likely cause failures in other areas. For example, if the registration is difficult to learn, users are likely to take longer to complete the task, exceeding efficiency targets, and be less accurate, failing in effectiveness.
Planning usability evaluations
Understanding specific targets for these usability goals also helps plan usability evaluation. The testing techniques selected may vary, depending on which of the characteristics you are most interested in. Some can be tested with early prototypes or even paper mockups, but others require working software or very high fidelity prototypes.
In planning usability evaluations, be sure that the most important characteristics are included, and tested in a realistic way.
Usability and user-centered design are iterative. The work proceeds in a cycle of hypothesis and evaluation, with a picture of users and design solutions to meet their needs building in richness and completeness with each iteration. The five Es (effective, efficient, engaging, error tolerant, easy to learn) provide the practitioner with a set of characteristics which can be used to organize and analyze information from users. They offer trace-ability from initial information-gathering through requirements setting and finally in evaluation. This might allow the understanding of the specific needs around each characteristic to grow, or be an opportunity to confirm whether the user requirements were chosen correctly in the early stages of the project. In either case, they let you go beyond "ease of use" in a practical way and help make it easier to make products more usable.
This paper was published in the Proceedings of the 48th Annual Conference, Society for Technical Communication, 2001
The URL for this article is: http://www.wqusability.com/articles/more-than-ease-of-use.htmlWhitney Quesenbery works on user experience and usability with a passion for clear communication. She is the co-author of Storytelling for User Experience from Rosenfeld Media. Before she was seduced by a little beige computer, Whitney was a theatrical lighting designer. The lessons from the theatre stay with her in creating user experiences. She can be reached at www.WQusability.com