A system and method for providing continuous confidence-based authentication. The present invention may be implemented in an incremental access authentication system for controlling access to secured data having various levels of security. During the course of a conversational session between user and machine, a confidence-based authentication system according to the present invention will periodically analyze the input speech of a user interacting with the system to compute a "confidence measure" for the validity of an original identity claim i provided by the user at the commencement of the dialog session. The "confidence measure" computation process according to the present invention is seamlessly integrated into the incremental access authentication system so that the system can tailor its interaction with the user based on its confidence in the original identity claim.