October 1995

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Introduction

Graphical user interfaces (GUIs) have become the user interface of choice. Yet despite the GUI's popularity, surprisingly few programs exhibit good interface design. Moreover, finding information explaining what constitutes a good and intuitive interface is exceedingly difficult. In this article, I describe the basic rules for all good interfaces -- the cardinal do's and don'ts.

However, before starting in on what constitutes good design, I need to explain the causes of bad design. This way, if you are tempted to deviate from the tried and true, you'll know where the wrong path leads -- and, I hope, get back to good design.

Design Mistake 1: Forgetting the User Developers often design for what they know, not what the user knows. This age-old problem occurs in many other areas of software development, such as testing, documentation, and the like. It is even more pernicious in the interface because it immediately makes the user feel incapable of using the product. Avoid this error diligently.

Design Mistake 2: Failing to Give the User Control The GUI designer's predilection for control is evident in applications that continually attempt to control user navigation by graying and blackening menu items or controls within an application. Controlling the user is completely contradictory to event-driven design in which the user, rather than the software, dictates what events occur. As a developer, if you are spending a lot of time dynamically graying and blackening controls, you need to re-examine your design approach and realize that you may be controlling the user, who may not want to be controlled. As business changes at a faster pace, flexibility in user interfaces will become a key enabler for change. Allowing the user to access the application in ways that you never dreamed can be scary, but satisfying, for you as a developer and empowering for the user.

Design Mistake 3: Providing Too Many Features at the Top Level Examine a VCR built in 1985, and then examine one built in 1995. You will see a startling difference in the interface of the two models. The model built in 1985 will have an abundance of buttons readily available on the faceplate of the unit, many of which will remain a mystery since the manual was lost years ago. The 1995 model will have only a few buttons for the key features that people use: play, fast forward, reverse, stop, and eject. The newer model will probably have even more features than the model built a decade before, yet the features will be cleverly tucked away behind a drop-down panel or sliding door, accessible when needed but not staring the user in the face.

Likewise, you should ensure that features used frequently in an application are readily available. Avoid the temptation to put everything on the first screen or load the toolbar with rarely used buttons. Do the extra analysis necessary to find out which features can go behind the panel instead of on the faceplate.

GUI Successes

Now, let's discuss some GUI successes. Successful GUIs share many common characteristics. Most importantly, good GUIs are more intuitive than their character-based counterparts. One way to achieve this is to use real-world metaphors whenever possible. For example, an application I recently examined used bitmaps of the Visa and MasterCard logos on buttons that identified how a customer was going to pay. This graphical representation was immediately intuitive to users and helped them learn the application faster.

Another important characteristic of good GUIs is speed, or more specifically, responsiveness. Many speed issues are handled via the design of the GUI, not the hardware. Depending on the type of application, speed can be the make-or-break factor in determining an application's acceptance in the user community. For example, if your application is oriented toward online transaction processing (OLTP), slow performance will quickly result in users wanting to abandon the system.

You can give a GUI the appearance of speed in several ways. Avoid repainting the screen unless it is absolutely necessary. Another method is to have all of the field validations occur on a whole-screen basis, instead of on a field-by-field basis. Also, depending upon the skills of the user, it may be possible to design features into a GUI that give the power user the capability to enter each field of each data record rapidly. Such features include mnemonics, accelerator keys, and toolbar buttons with meaningful icons, all of which allow the speed user to control the GUI and rate of data entry.

Do's And Don'ts Every good developer should have the goal of designing the best GUIs possible. But how does a developer make this goal a reality? By following sound, proven GUI design principles such as those listed in the following sections.

Like any good professional, YOU need some rules for repeatable successful designs. We have used the principles offered here for work with our own customers and have taught more than 20,000 GUI-design students nationally and internationally. These principles should help you as well.

Design Principle 1: Understand People Applications must reflect the perspectives and behaviors of their users. To understand users fully, developers must first understand people because we all share common characteristics. People learn more easily by recognition than by recall. Always attempt to provide a list of data values from which the user can select, rather than having the user key in values from memory. The average person can recall about 2,000 to 3,000 words, yet can recognize more than 50,000 words.

Design Principle 2: Be Careful of Different Perspectives Many designers unwittingly fall into the perspective trap when it comes to icon design or the overall behavior of the application. I recently saw an icon designed to signify "Rolled Up" totals for an accounting system. To show this function, the designer put a lot of artistic effort into creating an icon resembling a cinnamon roll. Unfortunately, the users of the system had no idea what metaphor the icon was supposed to represent even though it was perfectly intuitive from the designer's perspective. A reserved icons table containing standard approved icons, such as the one shown in Figure 1, will help eliminate these problems.

Figure 1 - Reserved Icons

Design Principle 3: Design for Clarity GUI applications often are not clear to end users. One effective way to increase the clarity of an application is to develop and use a list of reserved words. A common complaint among users is that certain terms are not clear or consistent. I often see developers engaging in spirited debates over the appropriate term for a button or menu item, only to see this same debate occurring in an adjacent building with a different set of developers. When the application is released, one screen may say "Item," while the next screen says "Product," and a third says "Merchandise", when all three terms denote the same thing. This lack of consistency ultimately leads to confusion and frustration for users.

Figure 2 gives an example of a list of reserved words. An application development group might complete and expand the table with additional reserved words.

Figure 2 - List of Reserved Words

Design Principle 4: Design for Consistency Good GUIs use consistent behavior throughout the application and build upon a user's prior knowledge of other successful applications. When writing software for business applications, provide the user with as many consistent behaviors as possible. For example, both the Embassy Suites and Courtyard Marriot hotel chains are growing rapidly due to their popularity among business travelers who know that they will be provided with a familiar room and a consistent set of amenities. The bottom line is that business travelers are not looking for a new and exciting experience in each new city. Business users of your software have similar needs. Each new and exciting experience you provide in the software can become an anxiety-inducing experience or an expensive call to your help desk.

Design Principle 5: Provide Visual Feedback If you've ever found yourself mindlessly staring at the hourglass on your terminal while waiting for an operation to finish, you know the frustration of poor visual feedback. Your users will greatly appreciate knowing how much longer a given operation will take before they can enjoy the fruits of their patience. As a general rule, most users like to have a message dialog box with a progress indicator displayed when operations are going to take longer than seven to ten seconds. This number is highly variable based on the type of user and overall characteristics of the application.

Design Principle 6: Be Careful With Audible Feedback Last week, I had the opportunity to ride in elevators in which a pleasant voice informed riders which floor they were on. The building was fairly new, and at first, employees thought the voice was cute. After six months of traveling floor to floor, employees are sure to ignore the voice and see it as more of an annoyance than a help. The same thing can happen with your GUls, except the annoying sounds are not contained within the walls of an elevator, but instead are available to everyone within earshot of the worker's cubicle. Put sound on a few hundred workstations, and a real cacophony emerges in the open-air cubicle environment. However, audible feedback can be useful in cases where you need to warn the user of an impending serious problem, such as one in which proceeding further could cause loss of data or software. Allow users to disable audio feedback, except in cases when an error must be addressed.

Design Principle 7: Keep Text Clear Developers often try to make textual feedback clear by adding a lot of words. However, they ultimately make the message less clear. Concise wording of text labels, user error messages, and one-line help messages is challenging. Textual feedback can be handled most effectively by assigning these tasks to experienced technical writers.

Design Principle 8: Provide Traceable Paths If your users ever say something akin to, "I don't know how I got to this window, and now that I'm here, I don't know how to get out," then you have not provided a traceable (or, in this case, retraceable) path. Providing a traceable path is harder than it sounds. It starts with an intuitive menu structure from which to launch your specific features.

You must also identify areas where you can flatten the menu structure and avoid more than two levels of cascading menus. Providing a descriptive title bar within each dialog box greatly helps to remind users what menu items or buttons were pressed to bring them to the window now in focus.

Design Principle 9: Provide Keyboard Support Keyboards are a common fixture on users' desktops and provide an efficient means to enter text and data. With the introduction of GUI applications, we often assume users will embrace a mouse as the primary interactive device. Using a mouse can become time-consuming and inefficient for the touch typist or frequent users of an application.

Keyboard accelerators can provide an efficient way for users to access specific menu items or controls in a window. The accelerators used should be easy to access and limited to one or two keys (such as F3 or Ctrl-P). Keyboards have limitations in the GUI world, such as when trying to implement direct-manipulation tasks like drag and drop, pointing, and re-sizing.

In contrast, you will always find a smaller set of users who are not touch typists and hence embrace the mouse as a point-and-click nirvana. The result is that you need to provide complete and equal keyboard and mouse support for all menu and window operations.

Design Principle 10: Watch the Presentation Model A critical aspect that ties all these facets of the interface together is the interface's look and feel. The look and feel must be consistent. On the basis of users' experiences with one screen or one dialog box, they should have some sense of how to interact with the next screen or control.

Searching the interface model for good design and continuity is very important. The model should involve careful decisions, such as whether the application has a single or multiple document interface. The model also will validate how users perform the main tasks within the application.

Identifying the appropriate presentation for the application greatly facilitates the subsequent windows being developed since they will have a common framework in which to reside. On the other hand, if you do not define the presentation model early in the design of your GUI, late changes to the look and feel of the application will be much more costly and time-consuming because nearly every window may be affected.

Design Principle 11: Use Modal vs. Modeless Dialogs Appropriately When we need input from the user, we often use a modal dialog box. Using modal dialogs has long been shunned by many developers as too constraining on the user. However, modal dialogs do have many uses in complex applications since most people only work on one window at a time. Try to use modal dialogs when a finite task exists. For tasks with no fixed duration, modeless dialogs are normally the preferable choice with a major caveat: Try to keep the user working in no more than three modeless windows at any one time. Go beyond this magical number and the support-desk phones will start ringing, as users spend their time managing the open windows rather than concentrating on their tasks. Use the table in Figure 3 to determine the appropriate use of dialog boxes and windows.

Figure 3 - When to Use Dialog Boxes or Windows

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