Chapter 1. User Interface Design Principles
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## Chapter 1. User Interface Design Principles

A user interface is simply the interface between an application and the user of an application. The primary goal of a user interface is to help user interface designers easily create applications that increase user effectiveness and satisfaction. By following the guidelines presented in this OSF/Motif Style Guide, you can create applications that are well designed and easy to use. These guidelines pertain to all sorts of applications, from spreadsheets and word processors to CAD tools.

To be effective, a user interface allows a user to interact with an application simply and naturally. Successful user interface designers keep the user in mind while designing an application. Keep the user in mind by following these two principles:

- Know the user.

- Empower the user.

The user of an application, above all, wants to get the job done. A user interface needs to be designed so the user can quickly and easily complete the tasks. Users want mastery over an application. Make it simple for the user to master the basics of your application. At the same time, you can include advanced methods of interaction with the application, shortcuts for the user. Users are curious and exploratory. They will find these shortcuts as they use and master the application. Such shortcuts need not be as intuitive as the regular methods of interaction.

This chapter discusses the following guidelines for creating user interfaces that are consistent and easy to use. Because of the particular nature of your client application, component, or window manager, or in response to customers' needs, you may not be able to apply all of these principles all of the time.

- Adopt the user's perspective.

- Give the user control.

- Use real-world metaphors.

- Keep interfaces natural.

- Keep interfaces consistent.

- Communicate application actions to the user.

- Avoid common design pitfalls.

## Adopt the User's Perspective

Effective design starts with adopting the user's point of view, which is often difficult to do. Application designers tend to see an application as the implementation of functions. In contrast, the user sees an application in terms of its interface.

Good design is rooted in an understanding of the user's work. A well-designed application solves users' problems, makes their work easier, and offers them new capabilities. The two most effective ways to understand the user's work are to involve users in the design and to be a user yourself.

Input from users can help determine both appropriate functions and methods for presenting them. Involve users as early as possible in the design process because, as the design progresses and the schedule closes in, the possibilities for design change decrease.

You do not need a working prototype to involve users. In fact, you can even involve users while you are writing specifications. At this stage, you can watch users work in order to understand the environment in which your application will be used. Talk to these users about their work, their current tools, and their goals for new tools.

For example, if you are designing software to create and display charts and graphs during meetings, you might attend meetings at various customer sites, see how charts and graphs are currently used in meetings, and interview meeting participants to learn what they would like to see in a new tool. Once you have a working prototype of your application, invite users to test it to see if your interface meets the goals you established for it.

Try to use your application in real situations. Using an application can provide critical insights into user interface problems. Acquiring experience with the application can be difficult and time consuming, but it is a worthwhile exercise. Before you even create the interface for your application, you can use similar applications, even competitive products, to help you understand the user's tasks.

## Give the User Control

Users want and need to be in control of the tools they use to perform their work. The user can be in control when an application is flexible and uses progressive disclosure.

### Keep Interfaces Flexible

Providing multiple ways for users to access application functions and accomplish their tasks increases their sense of control. Flexibility enables users to select the best method of accessing a function based on the criteria they choose: experience level, personal preference, unique situation, or simply habit. For example, a user can access a function through a Pulldown Menu, direct manipulation of an object, a mnemonic key press, or a keyboard accelerator.

Your application should also be configurable. Allowing users to configure settings and select personal preferences enhances their sense of control and encourages them to take an active role in understanding your product and how it works. To be effective, the configurability of your application needs to be easily accessible.

### Use Progressive Disclosure

Design your application so that the necessary and common functions are presented first and in a logical order. Make the more sophisticated and less frequently used functions hidden from immediate view, but still available. For example, use a DialogBox to hide settings that are not accessed often.

Decisions about the placement of functions are not easy to make. From the implementation standpoint, all functions are important. Often, however, a relatively small number of functions account for the majority of use. Make sure that these important functions are prominently featured in the presentation of the interface. Also remember that they can be prominent only if other functions are hidden.

## Use Real-World Metaphors

A good user interface allows the user to transfer skills from real-world experiences. For example, PushButtons push, and Scales slide. This makes it easier for the user to infer how to use an application. When you design a new component, consider how a similar real-world control performs to incorporate the metaphor into the new component. Real-world metaphors can extend to groups of components as well, especially when making a computer-based user interface to replace a mechanical user interface.

### Allow Direct Manipulation

Users need to be able to directly manipulate elements of the user interface and their applications. For example, the user needs to be able to directly scroll Text with a ScrollBar, rather than using a keyboard-driven command. Direct manipulation simulates the real world where the user employs tools to perform tasks on physical objects. Users control applications by directly manipulating graphical components similar to real-world controls, rather than entering a command on a command line. Direct manipulation reduces the amount of information the user needs to memorize.

Direct manipulation connects an action to an observable response from a component. Using direct manipulation, the user gets an immediate visible result from each action.

The direct manipulation model is an object-action model. That is, you first select an object or group of objects, then you perform an action on the selected objects. An object-action model allows the user to see what elements will be acted on before performing an action. It also allows multiple actions to be performed successively on the selected elements.

Although it is important to allow direct manipulation of the objects in your application, you must also support methods for interacting with your application by keyboard-only users. These methods can also be used by advanced users to perform some tasks more quickly.

### Provide Rapid Response

Make your application respond to input as rapidly as possible. The immediacy of the visual response is crucial to the experience of direct manipulation. When using components, provide the application's response immediately and in proportion to the component's actions. The application must also have a consistent speed of response. Delays, disproportionate responses, or inconsistent responses can render an otherwise well-designed application unusable. Performance problems make it difficult for the user to concentrate on the task at hand.

### Provide Output as Input

Another feature of direct manipulation is that the output of one part of an application or the output of the application itself is also available as input. For example, if one action produces a list of filenames, another action can select them for use elsewhere.

The user manipulates objects by locating them and clicking on them rather than typing in their names. Design so that the only time the user needs to type a name is to create an object. A well-designed application reduces the amount of information the user needs to memorize to perform tasks.

## Keep Interfaces Natural

You can extend the concepts of giving the user control and using real-world metaphors to arrange your application so that tasks flow naturally. Users need to be able to anticipate the natural progression of each task; through this anticipation, they are able to complete tasks more quickly.

Each screen object needs to have a distinct appearance that the user can easily recognize and quickly understand. At the same time, the style of the interface needs to graphically unify these elements and ensure a consistent and attractive appearance at any screen resolution.

### Make Navigation Easy

Make navigation easy by providing a straightforward presentation of the overall work area and the mechanisms for moving through it. Moving easily and quickly within the work area gives the user a sense of mastery over the application. For example, ScrollBars are an effective way to indicate the position of the current view in relation to an area as a whole. In addition to providing positional feedback, ScrollBars allow the user to move through the area.

Arrange elements on the screen according to their use; an optimal arrangement assists the user's decision-making processes and reduces the possibility of errors. The best approach for arranging screen elements according to use is to involve users in the arrangement process. Present screen objects in an orderly, simple, and uncluttered manner.

Reduce mouse movement to simplify the actions of the user. For example, place secondary DialogBoxes near their parent DialogBox so that when the secondary DialogBox appears, the mouse pointer is over the default PushButton, unless the user needs to see the contents of the original DialogBox. Reducing mouse movement helps make an interface natural because, from the user's point of view, work involves a stream of thoughts, intentions, and tasks (some predefined and some that become apparent during the process) that all relate to some desired outcome or accomplishment. This stream of thought is disrupted when the user has to make unnecessary mouse movements, open and close DialogBoxes, or search for commands.

### Provide Natural Shades and Colors

Minimize the contrast between screen objects in order to direct the user's attention. Appropriate use of contrast helps the user distinguish screen objects against the background of a window. Very dark screen objects on a light background, very bright objects on a dark background, and bright colors all command the user's attention. If there are many objects with strong contrast or bright colors on the screen, the user will have difficulty knowing where to look first because all these objects compete equally for attention.

Use color as a redundant aspect of the interface; that is, use it to provide additional differentiation among screen objects. Differentiation also comes from the shape and size of the screen objects. For example, in many parts of the world, stop signs are red octagons. You recognize the stop sign by both its shape and color.

## Keep Interfaces Consistent

The main purpose of the OSF/Motif Style Guide is to ensure consistency. Consistency is important both among applications and within a single application. Consistency helps the user transfer familiar skills to new situations. The user can apply the knowledge learned from one application to another application, reducing the amount of learning and subsequent recall. Consistency within applications facilitates exploration of new functions. When components work in a manner that is consistent with other components, the user will be less afraid to try new functions. The new functions will seem familiar, comfortable, and appropriate. The guidelines in the OSF/Motif Style Guide allow you to create applications that are consistent in a diverse market and that help your applications succeed in the marketplace.

Intraapplication consistency means the following:

- Similar components operate similarly and have similar uses.

  For example, because Pulldown, Popup, and Option Menus are similar components, their operation and use should be similar. Choosing the proper component is described in Section 6.1, "Choosing Components." Component interaction is described in the reference section for each component and also in Section 6.3, "Interaction."

- The same action should always have the same result.

  For example, pushing the top arrow in a ScrollBar should always move the ScrollBar up. Interaction is described in Section 6.3, "Interaction."

- The function of components should not change based on context.

  For example, clicking a button should always perform the same action. Note that even though the action is the same, the result of the action can depend on context. A button in a file editor can begin editing one of a number of files. The button need not always edit the same file; rather, its consistent action is to edit the selected file. Interaction is described in Section 6.3, "Interaction."

- The position of components should not change based on context.

  Components should not generally be added and removed as needed. This makes it difficult to quickly find the desired component. Instead, you should make unneeded components nonfunctional and indicate this by deemphasizing (graying out) their labels. Component layout is described in Section 6.2, "Layout."

- The position of the mouse pointer should not warp.

  The location of the mouse pointer should be determined by direct manipulation and should not be positioned arbitrarily by the application. Positioning the mouse pointer by the application causes the user to lose track of the pointer. Warping the pointer also causes problems with tablet style pointing devices that rely on absolute pointer positioning. The input model is described in Chapter 2, "Input Models."

Consistency among applications increases the user's sense of mastery. Experience with one application can be readily applied to another application, creating a positive transfer of knowledge. The task at hand, rather than learning a new application, becomes the focus of a computer session. When applications work in a manner that is consistent with other applications, users enjoy a feeling of immediate confidence in their ability to master the new program. Also, they are pleasantly surprised when trying new functions because, although new, the functions seem familiar.

Interapplication consistency means the following:

- Components look familiar.

  This does not mean that components look exactly the same, but that the internal layout of components should be the same. Elements of appearance such as color, size, and thickness of beveled edges are less important to application interoperability. Component design and layout are described in the reference section for each component and also in Section 6.4, "Component Design."

- Interaction is familiar.

  When interaction is different among applications, it confuses the user and makes it difficult to concentrate on the task of the application. This applies to the behavior of components, input methods, selection models, and keyboard navigation. Interaction is described in Section 6.3, "Interaction."

- Components are organized in a familiar manner.

  The user needs to be able to quickly find the proper component for each task. Organizing the components according to consistent guidelines helps the user do this. Application layout guidelines are described in Section 6.2, "Layout."

## Communicate Application Actions to the User

Effective applications let the user know what is happening with the application, but without revealing implementation details. Proper communication between the user and the application increases user satisfaction. There are three guidelines for communicating from the application to the user: provide feedback, anticipate errors, and provide warnings.

### Give the User Feedback

Feedback lets users know that the computer has received their input. Give users feedback whenever they have selected a component or Menu item by highlighting the component or Menu item in some way. In addition, if certain operations take more than a few seconds, you should let the user know that the computer is working on that operation by providing a message or by changing the pointer to a working pointer.

### Anticipate Errors

Anticipate the errors that are likely to occur. By anticipating errors, you can avoid them in your design, enable the support of recovery attempts, and provide messages informing the user of the proper corrective action. For example, one technique for avoiding excessive error messages is to dim interface components when they cannot be used.

Context-sensitive help aids understanding, reduces errors, and eases recovery efforts. Help information text needs to be clear, concise, and written in everyday language. Help information needs to be readily accessible and just as readily removable.

Many users are most comfortable with learning how to use software applications when they use a natural, trial-and-error method. An undo function supports learning by trial and error by minimizing the cost of errors. An undo function allows the user to retract previous actions, and fosters a spirit of exploration and experimentation that is essential.

### Use Explicit Destruction

Explicit destruction means that, when an action has irreversible negative consequences, it should require the user to take an explicit action to perform it. For example, while a worksheet can be saved simply by clicking on a Save PushButton, erasing the worksheet should require clicking on an Erase PushButton and answering a warning question like `"Are you sure you want to erase this worksheet?"` with a button click in the warning DialogBox.

Warnings protect the user from inadvertent destructive operations, yet allow the user to remain in control of the application. Warnings also encourage the user to experiment without fear of loss. Operations that can cause a serious or unrecoverable loss of data should warn the user of the consequences and request explicit confirmation.

## Avoid Common Design Pitfalls

The process of achieving good design presents many challenges and potential pitfalls. The following guidelines can help you avoid common pitfalls:

- Pay attention to details.

  The details of an application express the sense of craft that you applied to the application. The details of an elegantly designed interface both please users and facilitate their work. For example, aligning the PushButtons of two related and overlapping DialogBoxes makes it easier for the user to activate new settings in an apparently seamless operation. Consistent capitalization of Menu items and DialogBox labels is a design detail that reduces textual distractions for the user.

- Do not finish prematurely.

  A common design pitfall is assuming too early that a design is complete. This tendency is aggravated by schedule pressures and difficulty in pinpointing the inadequacies of a design. While it is important to begin designing early, it is also important to allow for redesigning for as long as possible. The first design of an application is not a solution but a fresh perspective from which to view interface design problems.

- Design iteratively.

  Interface design is best done iteratively. The development cycle of implementation, feedback, evaluation, and change avoids errors by allowing for early recognition and correction of unproductive designs.

- Start with a fresh perspective.

  Avoid the temptation to convert existing software by simply translating it to a new style of interface. Because direct manipulation changes the way the user works, a simple one-to-one translation is unlikely to be successful. Command line applications that are converted to direct manipulation need to be extensively reconsidered and revised. The structure of the function hierarchy and presentation needs to be completely redesigned.

- Hide implementation details.

  User interfaces need to hide the underlying software and present a consistent interface to the user. A good user interface does not allow implementation details of the application to show through; it frees the user from focusing on the mechanics of an application.

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