The vocabulary used for spoken input should be constructed so that only a few options are needed for any transaction.
Additional Information:
To increase the likelihood that a user's valid entries are correctly identified by the system, the user's vocabulary should be predictable. This does not necessarily mean that the vocabulary must be small, though recognition systems that can only accommodate small vocabularies are more prevalent and less expensive. A vocabulary is predictable when a user's choice of inputs at any given time is small, so that the system will be more likely to make a correct match in interpreting an entry.