A system for measuring heart rate variability (HRV) comprising 3 sub-systems: a data collection sub-system, a data analysis sub-system, and an output sub-system. A patient is connected to a heart monitoring device such as an ECG and the data collection sub-system records the patients heart beats, and an ECG chart is produced from which the patient's HRV value is derived by the data analysis sub-system. The present invention obtains the HRV value through calculation of a new parameter called relative density (RD). In accordance with the inventive method, data points are generated from the peak interval data of measured heart beats and the HRV relative density parameter (RD) is calculated by correlation between two subsets of data points.