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Constraints on measuring heart rate and period sequentially through real and cardiac time.

It is generally assumed that either heart rate or heart period can be used to measure phasic cardiac activity changing either from beat to beat or from second to second. However, only rate analyzed in real-time units and only period analyzed in cardiac-time units have the important property that the arithmetic mean of the unit estimates is free to vary and yields an average equal to the definitional estimate based on all units.