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Voice User Interface Design: Minimizing Cognitive Load

This chapter helps you deal with cognitive challenges involved in voice user interface design, including conceptual complexity, memory load, and attention. As with all design guidelines, they must be applied with careful consideration of context.
This chapter is from the book

This chapter is from the book

Cognition is the processing of information from the world around us. It includes perception, attention, pattern matching, memory, language processing, decision making, and problem solving. Cognitive load is the amount of mental resources needed to perform a given task.

All user interfaces make cognitive demands on users. Users must master special rules of system use, learn new concepts, and retain information in short-term memory. They must create and refine a mental model of how the system works and how they should use it. Systems that use purely auditory interfaces further challenge human memory and attention because they present information serially and non-persistently.

Successful user interface designs must respect the limitations of human cognitive processing. If a design requires the user to hold too many items in short-term memory or to learn a complex set of commands too quickly, it will fail. This chapter describes a number of guidelines for minimizing the cognitive load on users of spoken language interfaces.

There are three cognitive challenges you should consider as your design progresses:

  1. Conceptual complexity: How complex are the new concepts callers must learn? How well do new mental structures match concepts and procedures that users are already familiar with?

  2. Memory load: How much information must callers hold in their short-term memory? How much new material (e.g., commands, procedures) must they learn?

  3. Attention: Is it easy for the caller to attend to the most salient information? Will callers' attention be divided? If they are momentarily distracted (e.g., while driving), can they seamlessly continue their interaction with the system when they are ready?

The following sections discuss each of these potential challenges and present guidelines for handling them.

9.1 Conceptual Complexity

Conceptual complexity is, in part, a product of the new concepts the user must learn and the inherent complexity of those concepts. However, understanding the cognitive challenges goes beyond counting concepts and measuring their complexity. It is also a matter of understanding human capabilities (what is hard and what is easy for humans to understand and learn) and understanding the context in which users will operate (e.g., how the current application will mesh with existing user knowledge, skills, expectations, and mental models).

In this book we do not try to present a theoretical framework that allows precise prediction of the difficulty of particular design decisions. The necessary knowledge to create such a theory is incomplete. Instead, we present a set of guidelines that will help you minimize the cognitive challenges for your callers. This section covers the following guidelines:

  • Establish constancy: Create constants, or universal commands, that are always available to the caller regardless of context. Universal commands should be designed to let callers recover from problems or receive help using the system.

  • Ensure consistency: Do similar tasks in similar ways throughout the application. For example, whenever a caller traverses different kinds of lists in a large portal application, make the same set of list traversal commands available. In this way, you minimize the amount of new material the caller must learn.

  • Set the context: Set the context for your callers. Make it clear why the system is taking specific actions. Appeal to their world knowledge and expectations in order to simplify new concepts they must understand.

9.1.1 Constancy

Graphical user interfaces take advantage of the ability to display information, often lots of it at the same time, on the computer screen. For example, many GUIs display a toolbar (see Figure 9-1), usually at the top of the screen. This toolbar, which typically consists of a row of icons representing actions, provides both a visual reminder of popular actions and a physical means to initiate them.

09fig01.jpgFigure 9-1. A GUI toolbar displays unchanging icons.

The toolbar is constant: It remains on the screen, and the icons never change. The toolbar's constancy reduces the user's need to memorize a set of actions and commands.

Similarly, VUIs can achieve constancy by using universals: a small set of spoken commands that are always available regardless of context (see section 5.2.2). After callers learn the universal commands, they can use them at any point as well as in future calls. These commands become, in effect, a mental toolbar of always available actions (see Figure 9-2).

09fig02.gifFigure 9-2. Callers form a "mental toolbar" of spoken universals.

It is not practical to expect a caller to learn more than a very small number of universal commands. This number may become slightly larger if a standard set of universals is established and widely used throughout the speech industry.

Given that the number of universal commands should be small, it is best to associate the commands with functions that a caller can use to get out of trouble—for example, asking for additional help or instruction, moving to a different part of the application, or requesting to speak to a live agent. Successful use of such universals should improve transaction completion rates, automation rates, and user satisfaction.

You should choose command words or phrases for universals that are intuitive and easy to remember (for example, "help"). The commands should have the same meaning, no matter when they are spoken. For example, saying "Help" should always mean that the caller wants more detailed instruction about what can be done at the current point in the dialog. The instruction the caller hears in response will vary depending on the current context, but the universals should always be available.

Two standards bodies have been looking into universals: the Telephone Speech Standards Committee (TSSC 2000) and the European Telecommunications Standards Institute (ETSI 2002). Both committees have solicited input from the design and development community about its experience with universals. The two groups have conducted experiments to see what terminology occurs most naturally to users when they want to elicit the universal behaviors. Both committees have reached similar tentative conclusions.

The following list shows the set of universals that we recommend for all applications. The word or phrase in brackets is the actual command callers would use. Our choice of universals is based on the results of the two standards committees as well as our own experience with deployments. In the future, if an industrywide standard is accepted, we support conformance. The entire industry, and our callers, will benefit if a consistent set of universals is used for all applications.

  • Clarification universals

    • [help]: Provide help or additional instruction about the current dialog state.

    • [repeat]: Repeat the most recently played prompt.

  • Navigation universals

    • [main menu/start over]: Return to the beginning of the application (following any login process).

    • [go back]: Back up to the preceding step.

  • Termination universals

    • [operator]: Transfer to an operator or customer service agent.

    • [good-bye]: Allow the caller to say "Good-bye" and respond appropriately so that the caller is comfortable hanging up.

The good-bye command is included because analysis of deployment data has shown that callers say it even when they are never told it is an available command. Usability studies have suggested that many callers are more comfortable saying good-bye, rather than just hanging up, especially when transactions are involved. It gives them confidence that hanging up does not prevent their transaction from being completed.

In general, callers must be taught about universal commands, or else they won't use them. One approach is to mention the help command in the initial prompt, which the caller hears when first entering the system. Description of the other universals can be included as the last part of help prompts, as well as part of any error recovery prompts. For example, a banking application might have the following initial prompt:

  • Welcome to Western Valley Bank. If you ever have a problem while you're using this service, just say, "Help." Now, would you like to pay a bill, check your balance, or transfer money?

If the caller says "Help" while in the middle of an operation for obtaining an account balance, the system response might be the following:

  • Okay, here's some help. You requested an account balance, but I don't know which account. You can say, "Savings" or "Checking." Anytime you like, you can also say "Main menu" or "Operator."

9.1.2 Consistency

You can also reduce callers' cognitive load by giving careful attention to consistency. When you design dialogs, the idea is to let the caller do similar things in similar ways. For example, there may be a number of lists that can be traversed in the course of an application (e.g., the stocks in the caller's portfolio, outstanding buy or sell orders, the caller's stock watch list). Using the same traversal strategy lets callers carry over what they learn from traversing one list to the traversal of all other lists.

Consistency can be applied at many levels. Dialog strategies should be consistent, as should terminology. Grammar coverage also should be consistent; for example, if you use money-amount grammars in two places, don't allow "fifty bucks" in one place but not the other.

In general, callers learn from every interaction. Each exchange between callers and the system refines callers' mental model of the system. That model includes their view of the system's capabilities, the commands available to perform various tasks, the richness and flexibility of grammars, and so on. Being consistent helps make the caller's learning process more efficient and effective. Failing to be consistent contributes to learning inefficiency, leads to confusion, raises the out-of-grammar rate, and provides a much less satisfactory experience for callers.

Let's consider an example of list traversal. In the following dialog with a brokerage system, two types of lists are traversed: the list of companies in the caller's watch list and the list of outstanding orders for buys and sells. Notice that the caller uses the same set of terms for navigating each list.




What would you like to do?


Play my watch list.


International Business Machines, eighty two, up one point three six. Amazon, thirty six, down . . .




Ford Motor Company, ten point . . .




Charles Schwab Corporation, eleven . . .




What would you like to do next?


Tell me my open orders.


You have five open orders. Here's the first one: sell one hundred shares of Microsoft . . .




Buy two hundred shares of Cisco Systems at seventeen . . .




Buy five hundred shares of International Business Machines at eighty two . . .




What would you like to do next?

9.1.3 Context Setting

Context setting is another important concept that can be applied to the design of voice user interfaces to minimize cognitive load (Weinschenk and Barker 2000). Psychological research has shown that people can understand and remember information more easily when it is presented with the appropriate context. For example, consider the following passage (Bransford and Johnson 1973):

  • The procedure is actually quite simple. First you arrange things into different groups. Of course, one pile may be sufficient depending on how much there is to do. If you have to go somewhere else due to lack of facilities, that is the next step; otherwise you are pretty well set.

When asked to remember as many ideas in this passage as possible, subjects remembered only about three. However, when told beforehand that the passage is about laundry, they recalled twice as many ideas. Context helps people to associate new information with familiar concepts, easing cognitive load.

One way to set the context in a user interface is to use a metaphor. As discussed in Chapter 4, a metaphor is a familiar object or schema that is used to help facilitate understanding in another domain. You may recall the examples of the desktop metaphor and the shopping cart metaphor.

To investigate whether metaphors actually help users of voice interfaces, researchers at British Telecom conducted a study that compared three shopping applications equipped with a voice interface (Dutton, Foster, and Jack 1999). One system had no metaphor and simply described merchandise through a menu structure. Another had a department store metaphor in which the callers used a virtual elevator (with sound effects) to move from floor to floor. The system described the merchandise on display for that floor. A third system had a catalog magazine metaphor in which the system described merchandise presented in magazine pictures.

Callers rated the system with the department store metaphor significantly higher than the system with no metaphor. (The system with the magazine metaphor scored between these two.) In addition, callers were better able to navigate the system when a metaphor was present. These findings indicate that context setting through the use of a metaphor can improve user satisfaction and effectiveness.

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