The variability of variables influences conclusions in the measurement of biologic variables particularly if the variability of methodological variables are unidentified or of unknown range. Therefore prior the interpretation of Holter-results the interobserver variability of measured parameters (e. g. arrhythmias) and the system-dependent variables (e. g. reproducibility) have to be recognized. Measuring biologic variables like arrhythmias or symptoms using the Holter method the spontaneous variability of infrequent occurring phenomena has to be taken into account and also their relationship to the recording duration: Infrequent occurring phenomena demonstrate an enormous variability, which could be reduced by lengthening the recording duration beyond 24 hours, especially in the evaluation of an antiarrhythmic therapy to avoid a mimicked therapeutic effect. In symptomatic patients their symptoms can be related to arrhythmias or not if an ECG will be recorded during a typical symptomatic period. This depends on the frequency and on the duration of the symptoms, so that other methods than Holter-monitoring should be considered like event-recording, ECG-telephone-telemetry or electrophysiological investigations. In summary the variability of phenomena captured by the Holter method (e. g. arrhythmias, symptoms) is a major factor, which has to be taken into account to avoid unsatisfactorial use of the Holter method.