For each ECG parameter the median and the 2nd and 98th percentiles of the measurement distribution per age group and gender were determined. The 2nd percentile was taken as the lower limit of normal (LLN), the 98th percentile as the upper limit of normal (ULN).

The statistical treatment of interval durations (heart rate, QRS duration, etc.) is straightforward since they are measured in every single case of the study population. The various wave components within the QRS complexes, however, are of varying occurrence. For example, in the present adult material of 13,354 normal cases a Q in lead V2 is an uncommon finding. It was observed 113 times (0.8%). The median duration of these Q waves is 20 ms and the 98th percentile 65 ms. Clearly, this does not imply that in the normal lead V2 a Q wave is to be expected of such duration. Rather, if all 13,241 cases in which a Q wave is missing, i.e., where the Q duration is 0, are included in the calculation, the median Q duration will certainly be 0, as will be the ULN since the Q occurs in less than 2% of the population. This means that a Q wave must be considered to be absent in the normal lead V2. If the proportion of Q’s in the total population is high, like it is in lead III, the ULN will move toward the one calculated separately for the Q, but the median value will remain 0 as long as this proportion stays <50%. This approach was generalized to other leads and waves.