Predictability and Prompt Variations
Usability 101
Usability is all but predicated on predictability. Predictability is a fundamental component of a sound VUI design. Most everyone seems to appreciate this fact, yet using prompt variations, a common design practice, can sometimes compromise an application's basic predictability.
The Good Intention
In an apparent attempt to make dialogs seem less deterministic, many designers employ prompt variations. The practice is simple: instead of invariably using only one recording of a prompt for a particular functional event, two or more versions are recorded and used. For example, consider the single, one-size-fits-all greeting:
Mono-Prompt Version:
"Thanks for calling the ACME Company"
as opposed to the time-of-day-specific variable alternatives:
Multi-Prompt Version 1:
"Good morning! And thanks for calling the ACME Company"
Multi-Prompt Version 2:
"Good afternoon! And thanks for calling the ACME Company"
Multi-Prompt Version 3:
"Good evening! And thanks for calling the ACME Company"
Another, more prevalent practice is to vary error prompts. Note that this practice is different from the all-but-universal practice of employing escalating error prompts when the user experiences two or more consecutive errors. In other words, one of the following multi-prompt versions would be randomly played to the user when experiencing, say, an initial error. Again, consider the one-size-fits-all version:
Mono-Prompt Version:
"Sorry. Say again, please."
as opposed to the randomly sampled variable alternatives:
Multi-Prompt Version 1:
"Sorry. Would you repeat that?"
Multi-Prompt Version 2:
"My mistake. Please repeat."
Multi-Prompt Version 3:
"I missed that. Once again, please."
So What's the Problem?
So what is so horrible about a little variation? After all, doesn't variation help make the application seem less rigid, artificially structured and utterly deterministic? I'll answer each question individually.
Q: Is prompt variation a universally "bad" idea?
A: Not really.
Q: Does variation make applications seem less rigid, artificially structured and utterly deterministic?
A: Maybe.
However…
When considering the value or appropriateness of a practice such as prompt version variations, the single most important question is this:
"Does the practice make the application easier to use?"
If the answer is not measurably "yes," then the practice is ill advised. Note that measurability implies some behavioral variable such as "percent tasks complete" or "time to task completion." It would not include assessments such as, "Subjects seemed to prefer prompt variations" or even, "When asked, subjects reported a preference for prompt variations."
The Real Problem
The most a designer can hope for from prompt variations is that the user will perceive the dialog to be less rigid, artificially structured or completely deterministic. The fact remains, however, that the application is rigid, artificially structured and completely deterministic. In other words, the most the designer can hope for is to deceive the user. Unfortunately, the deception is extremely difficult to maintain.
Consider the time-of-day-specific alternative prompts. What happens when a user calls from another time zone and the system reads a situationally inappropriate greeting? True, one could use some generic default when the caller's ANI is unknown but what if the ANI is known, it just happens to be a cell phone and the caller is in a time zone other than his home time zone? No doubt, some sort of work around might be found but this seems like an awful lot of trouble just to maintain an illusion.
What about the error message variations? Wouldn't they be better than repeating the same dull prompt? It might be true that the first time they are heard, the user might get a sense of less rigidity and artificiality, but what will the repeat user think when he has heard all of the variations several times before? Will he still perceive the application to be less deterministic?
A Simple Rule
Prompt variations, by definition, attempt to make the experience of an IVR event unique. This in itself can make an application appear somewhat unpredictable. As a rule, the more unpredictable a system appears, the less usable it becomes.
Walter Rolandi is the founder and owner of the Voice User Interface Company in Columbia, S.C. Rolandi provides consultative services in the design, development and evaluation of telephony-based voice user interfaces (VUI) and evaluates ASR, TTS and conversational dialog technologies. He can be reached at wrolandi@wrolandi.com.