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A model is developed to describe and quantitate the error in R-R interval measurement due to the finite sampling frequency. The measured R-R interval is modeled as the sum of the true R-R interval and the error measurement. The first- and second-order statistics of the error are computed in order to investigate its influence on the heart-rate-variability (HRV) power spectrum. The statistics are found to be functions only of the ECG sampling frequency, and it is shown that the power spectrum of the error contributes an additive high-pass-filter-like term (colored noise) to the power spectrum of the HRV. The accuracy of the model is tested by a simulation procedure. The model indicates that the relative balance between the HRV and error power spectra is important and should be checked before any variability analysis of heart rate.