8.5 Design Principles

Based on analyses of data from calls to deployed systems, usability studies of deployed systems and systems in development, and carefully controlled laboratory studies, six core VUI design principles have emerged. These principles can be applied to optimize the design of dialog strategies, call flows, and prompts:

• Minimize the cognitive load: Minimize the short-term memory load on callers, the complexity of concepts that callers must understand, and the number of things they must learn in order to use the system successfully. Chapter 9 covers techniques for controlling cognitive load.

• Accommodate conversational expectations: Human-to-human conversation is guided by conventions and unconscious expectations. These same expectations are in effect when humans converse with machines (Reeves and Nass 1996). Adhering to these natural expectations not only helps an application flow more easily but also increases comprehension. Chapters 10 and 11 explain how to design prompts based on conversational principles.

• Maximize efficiency: Callers want speed and efficiency. The fewer the number of steps your dialog design requires, the greater the perceived efficiency of the phone call. Chapter 12 discusses efficiency.

• Maximize clarity: Clarity is always key. This applies to low-level details, such as clarity about the meaning of prompts, and high-level details such as clarity of the appropriate mental model for using the system. Chapter 12 covers clarity and methods of balancing efficiency and clarity appropriately when you must trade off one with the other.

• Ensure high accuracy: Recognition errors must be minimized, because they seriously degrade usability and caller confidence. Chapter 13 covers VUI design techniques for high accuracy.

• Gracefully recover from errors: When errors occur, callers often become confused and frustrated. If an automated system lacks quick and graceful recovery from errors, callers will refuse to complete tasks, either by hanging up or by requesting transfer to a live agent. Chapter 13 covers the types and causes of errors and describes techniques to help a caller seamlessly and gracefully recover.

Interestingly, these six principles, which have emerged largely from analysis of live data from phone calls made to spoken language systems, coincide closely with user interface design guidelines and principles developed in other fields of study, such as graphical user interface (GUI) design. See, for example, Shneiderman (1998) or Nielsen (1993). However, the way these principles are applied to VUI design is different from the way they are applied to other types of interfaces. Although the first principle (minimizing cognitive load) is an issue for all user interfaces, it is of special importance for auditory interfaces, given the cognitive challenges that result from their nonpersistent nature. The second principle (accommodating conversational expectations) is specific to voice user interfaces, although the accommodation of user expectations in general is certainly important for all user interfaces.