Other Guiding Principles
Consistency and symmetry are two guiding principles for designing voice caller interfaces. Other guiding principles include the following:
Callers learn-by-doing. Psychologists tell us that one of the best methods for learning how to do something is to do it. This "sink-or-swim" approach is especially effective when callers are assured that if they start to sink, they will receive help to get back on track. Do not present long tutorials at the beginning of a telephony application. Callers remember little of the specific instructions, and resent the time required to listen to the tutorial. Instead, encourage the callers to explore. Dialog designers encourage callers to explore the speech application in at least two ways:
Prompts suggest words that the caller may say to explore new portions of the application. This encourages the caller to jump off into "unknown waters." For example, the following informs the user of physical objects that can be controlled from the telephone:
"What action? Thermostat, lights, security, kitchen appliances, or alarm clock?"
Specify event handlers to help the caller when the speech-recognition system fails to understand what the caller says. This pulls the "sinking" caller back to the surface.
Establish specific performance criteria. Developers need to conduct usability tests and measure performance criteria to determine whether changes to an application have an overall beneficial or harmful effect. It is easy to be misled by a change that solves a specific problem yet has an overall harmful effect to a speech application.
Leverage the caller's knowledge. A caller familiar with the application domain should find the application easy to learn and natural to use. This occurs when the caller's conceptual model of the domain (the caller's understanding of the domain objects, relationships commands, and constraints from real life) match the application's conceptual model. Talk with several prospective callers before constructing the application; and use the terms, phrases, and names frequently used by the prospective callers.
Optimize the frequent case. Make it easy to do simple things and possible to do complex things. Just like airport security, fliers with no suspicious objects go to an express line, whereas fliers with suspicious objects are asked to go to a "trouble" line. Frequent fliers quickly learn how to avoid the trouble lines so they can get through the security check faster. Organize the call flow so callers can perform the simple and frequent tasks quickly and be routed to specific paths in the call flow for infrequent and difficult tasks.
For additional suggestions on how to improve the quality and experience of using telephony applications, see the author's book, VoiceXML: An Introduction to Building Voice Applications (Prentice Hall, 2002, ISBN: 0130092622).