  Although a system is described by a well-known set of equations leading to a
deterministic behavior, in the real world the value of a measurand obtained by
an experiment will mostly scatter. Accordingly, an uncertainty is associated
with that value of the measurand due to apparently random fluctuation. This
papers deals with the question why this discrepancy exist. Furthermore it will
be shown how the uncertainty of one individual observation is calculated and
consequently how the best estimate and its corresponding uncertainty
considerering auto-correlstions is determined.
