This is the case when the operator performs a series of replicate measurements in repeatability conditions of measurement (cf. VIM §3.6).
The arithmetic mean obtained from the equation is used as the best estimate of the input quantity Xi. The values of the individual observations xi,k iffer due to the random variations of the influence quantities. The variability of the observed values xi,k or, more specifically, their dispersion around their mean value is called experimental standard deviation and is noted :
We can deduce from it the experimental standard deviation of the average such as , that is to say:
In practice, the experimental standard deviation of the average is called uncertainty of repeatability.