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Symmetry
The principle of symmetry suggests that callers respond to prompts in the same style and wording used by the prompt. If the prompt is lengthy, the caller's responses tend to be lengthy. If the prompt is vague, then the caller tends to respond with vague answers. In order to encourage callers to say the words and phrases covered by a grammar, the prompt should be short, precise, and instructive. Paraphrasing the age old Golden Rule: "Do unto the caller as you would have the caller do unto you."
Designers specify two key items for each turn of an application-directed dialog:
Specify the grammar to contain the words and phrases that minimize the errors made by the automatic speech recognizer.
Formulate the prompts to encourage the caller to speak the words and phrases in the grammar.
After specifying the grammar, apply the principle of symmetry by encouraging the caller to parrot words, phrases, and patterns used within the prompt.
Prompt the caller with a menu of words from the grammar. The caller can respond by selecting one of the words from the menu rather than guessing what words are in the grammar. For example, suppose the grammar contains two words: "Green" and "gray." For example, avoid using the following prompt:
Prompt: What color?
Response: Chartreuse.
Instead, enumerate the key words from the grammar:
Prompt: What color? Green or gray?
Response: Green.
Selecting from a verbal menu is cognitively easier for the caller than selecting a word from one's mind. Verbal menus also decrease speech-recognition engine mismatches, which happen when callers say words not in the current grammar.
Prompt the caller with a pattern that the caller can mimic. Consider the following prompt to confirm values previously spoken by the caller:
Prompt: Do you want to fly from Boston to Chicago on Thursday?
Response: No, I want to fly from Austin to Chicago on Thursday.
Words in the pattern are useful to assign spoken values to field names. In the above example, the spoken word "Austin" is assigned to the departure_city because of the pattern word "from," whereas "Chicago" is assigned to the arrival_city field because of the pattern word "to."
Prompt the caller with a short question This encourages the caller to respond with a short response. For example, avoid using the following:
Prompt: Please say your first name.
Response: My first name is Jim.
Instead, use the following:
Prompt: Your first name?
Response: Jim.
Short responses are easier for the speech-recognition engine to recognize rather than selecting keywords from the caller's response. A short prompt may also avoid false expectations about what the speech-recognition engine can recognize.