The authors studied time-to-time variability of 100 patients with internal diseases in 100 ECG's over a period of 15 minutes. The variavirity were observed on eleven electrocardiographic items which were measured in 12 leads (I～III, (a)V(R)～(a)V(F), V(1～6)). The electrocardiograms were recorded by same technician, who carefully recorded in confomity to the description on the variability due to techinical and biologocal sources in Simonson's writing and were measured by one of the authors. Frequency distribution of differences between each two electrocardiograms of the same patients are shown in Figures (from 1 to 11) and 5% rejection limits of these
differences, which were calculated in use of the stochastics, were as follows : P duration (sec. ) : + 0.022 >X(0)> - 0.021 P-Q duration (sec.) : + 0.029 >X(0)> - 0.024 QRS duration (sec.) : + 0.018 > X(0)> - 0.013 R-R interval (sec.) : + 0.168 >X(0)> - 0.141 QT Ratio (%) + 10.01 >X(0)> - 7.89 QT(c) : + 0.032 >X(0)> - 0.022 P/PR segment : + 0.914 >X(0)> - 0.832 QT/TQ + 0.192 >X(0)> - 0.190 T/R V(5) : + 0.116 >X(0)> - 0.090 AQRS (front. plane) : + 13. 29°>X(0)> - 13.83° AT (front. Plane) : + 14.88°>X(0)> - 16.94°
