An implantable cardiac stimulus device (ICSD) comprising a canister housing operational circuitry and a lead electrode assembly including a plurality of electrodes disposed thereon, with the lead electrode assembly configured to couple to the canister to electrically couple the operational circuitry to the electrodes on the lead electrode assembly, the operational circuitry comprising: input means for receiving signals from the electrodes; detection means for comparing the received signals to a detection threshold and identifying crossings of the detection threshold by the received signals as raw detected events, the raw detected events being separated by raw intervals; characterized by the operational circuitry further comprising: waveform appraisal means for performing waveform appraisal on a raw detected event to determine whether it results from or is masked by noise and, if so, for marking the raw detected event as a suspect event and marking the raw intervals immediately before and after the suspect event as a suspect intervals, otherwise, for marking the raw detected event as a WA detected event; overdetection analysis means for performing overdetection analysis on WA detected events and intervals between pairs of WA detected events; interval correction means for combining two non-suspect intervals separated by a WA detected event into a single combined interval if the WA detected event separating the two non-suspect intervals is marked as an overdetection; interval certification means for certifying an interval between two detected events for use in rate calculation if neither detected event is marked suspect or overdetection, or the interval is a combined interval; and rate calculation means for calculating a rate of cardiac events that uses only certified intervals for the purpose of identifying treatable cardiac conditions. input means for receiving signals from the electrodes; detection means for comparing the received signals to a detection threshold and identifying crossings of the detection threshold by the received signals as raw detected events, the raw detected events being separated by raw intervals; characterized by the operational circuitry further comprising: waveform appraisal means for performing waveform appraisal on a raw detected event to determine whether it results from or is masked by noise and, if so, for marking the raw detected event as a suspect event and marking the raw intervals immediately before and after the suspect event as a suspect intervals, otherwise, for marking the raw detected event as a WA detected event; overdetection analysis means for performing overdetection analysis on WA detected events and intervals between pairs of WA detected events; interval correction means for combining two non-suspect intervals separated by a WA detected event into a single combined interval if the WA detected event separating the two non-suspect intervals is marked as an overdetection; interval certification means for certifying an interval between two detected events for use in rate calculation if neither detected event is marked suspect or overdetection, or the interval is a combined interval; and rate calculation means for calculating a rate of cardiac events that uses only certified intervals for the purpose of identifying treatable cardiac conditions. waveform appraisal means for performing waveform appraisal on a raw detected event to determine whether it results from or is masked by noise and, if so, for marking the raw detected event as a suspect event and marking the raw intervals immediately before and after the suspect event as a suspect intervals, otherwise, for marking the raw detected event as a WA detected event; overdetection analysis means for performing overdetection analysis on WA detected events and intervals between pairs of WA detected events; interval correction means for combining two non-suspect intervals separated by a WA detected event into a single combined interval if the WA detected event separating the two non-suspect intervals is marked as an overdetection; interval certification means for certifying an interval between two detected events for use in rate calculation if neither detected event is marked suspect or overdetection, or the interval is a combined interval; and rate calculation means for calculating a rate of cardiac events that uses only certified intervals for the purpose of identifying treatable cardiac conditions.