A computer readable storage medium configured to cause a system to perform operations of computing the standard deviation of a plurality of K−1 most recent heart beat intervals and a current heart beat interval (I1) from a plurality of consecutive heart beat intervals, from a sum (S1) of K most recent heart beat intervals squared and the sum (S2) of the K most recent heart beat intervals and a Kth oldest heart beat interval (I2). The computer readable storage medium calculates an updated sum (S1′) of S1 minus I2 squared plus I1 squared and the updated sum (S2′) of S2 minus I2 plus I1, and a square root of the sum of the ratio of S1′ to K and of the ratio of the square of S2′ to the square of K.
