An accurate way to determine the relation between QT intervals and their corresponding RR intervals is described. A computer algorithm measures the RR intervals using digital filtering and cross-correlating the QRS sections of consecutive waveforms. The QT intervals is calculated by choosing a section of the ECG that includes the T waves. At least 4000 pairs of QT-RR intervals are computed for each subject; a best-fit correlation function determines the relations between the QT and RR intervals. This technique makes it possible to establish a precise correlation between RR and QT in order to distinguish between control and SIDS babies.