An automated external defibrillator for use during cardiopulmonary resuscitation, comprising: an input of an electrocardiogram signal; a user interface having at least one of an aural instruction output and a visual display; an electrocardiogram analyzer in communication with the input and configured to determine a shockable cardiac rhythm in the presence of cardiopulmonary resuscitation - related signal noise artifact from the input; a confidence analyzer configured to determine a confidence level in the determined shockable cardiac rhythm from value pairs, a value pair including a first score (CLAS score) and a second score (FLATS score) wherein the first score is comprised of a score from an electrocardiogram buffer from the electrocardiogram signal filtered at a first center frequency and the second score is comprised of a second score from the same electrocardiogram buffer filtered parallel at a second center frequency different from the first center frequency; and wherein the electrocardiogram buffered first and second scores are compared to a predetermined decision surface which is constructed using databases of electrocardiogram signal data having cardiopulmonary resuscitation corruption noise; and a processor in communication with the user interface, the electrocardiogram analyzer and the confidence analyzer, the processor configured to execute software instructions to adjust a shock decision criteria if the confidence level is less than a predetermined confidence threshold, decide that a shock is to be delivered by an electrotherapy delivery circuit based on the adjusted shock decision criteria, and arm an electrotherapy delivery circuit responsive to a shock delivery decision. an input of an electrocardiogram signal; a user interface having at least one of an aural instruction output and a visual display; an electrocardiogram analyzer in communication with the input and configured to determine a shockable cardiac rhythm in the presence of cardiopulmonary resuscitation - related signal noise artifact from the input; a confidence analyzer configured to determine a confidence level in the determined shockable cardiac rhythm from value pairs, a value pair including a first score (CLAS score) and a second score (FLATS score) wherein the first score is comprised of a score from an electrocardiogram buffer from the electrocardiogram signal filtered at a first center frequency and the second score is comprised of a second score from the same electrocardiogram buffer filtered parallel at a second center frequency different from the first center frequency; and wherein the electrocardiogram buffered first and second scores are compared to a predetermined decision surface which is constructed using databases of electrocardiogram signal data having cardiopulmonary resuscitation corruption noise; and a processor in communication with the user interface, the electrocardiogram analyzer and the confidence analyzer, the processor configured to execute software instructions to adjust a shock decision criteria if the confidence level is less than a predetermined confidence threshold, decide that a shock is to be delivered by an electrotherapy delivery circuit based on the adjusted shock decision criteria, and arm an electrotherapy delivery circuit responsive to a shock delivery decision.