ECGs from the adult population were recorded at a sampling rate of at least 500 Hz, pediatric ECGs were recorded at 1200 Hz. For the pediatric ECGs, V3R was used instead of V3 and V7 instead of V5.

All ECGs were processed by the Modular ECG Analysis System (MEANS). MEANS has extensively been evaluated both by its developers and by others. The interval measurements make use of the common wave onsets and offsets of the 12 simultaneous leads together. The morphological measurements are taken from one averaged representative beat across the 12 leads.

All ECGs were visually checked for correct waveform recognition by MEANS. Those with waveform recognition errors (mainly due to excessive noise) were also removed from the dataset. Electrode interchanges were detected by a special feature in MEANS. After visual confirmation these ECGs were also discarded. 

Spatial parameters were calculated from vectorcardiographic X, Y, and Z leads. These leads can, in good approximation, be reconstructed from the standard ECG leads. Normal values are provided for two reconstruction methods, a regression-based method and a model-based method (inverse Dower). Normal values of the frontal P, QRS, and T axes calculated from leads I and aVF, are also provided. All axis computations are based on the net areas under the the waves of the contributing leads.