= Measurement repeatability
<Measurement repeatability> is <measurement precision> under a declared set of repeatability conditions. The VIM's <repeatability condition of measurement> includes the same procedure, operators, measuring system, operating conditions and location, with repeated measurements over a short time.<International vocabulary of metrology by JCGM>[VIM3entries2.20–2.21, definitions]{ref} The complete measurement cycle must be specified before its repeatability can be assessed.
For an original length-comparator study, one operator measures the same reference block using the same <differential length comparator> over a short session. Three readings without disturbing the setup happen to be $20.000,20.000,20.000\,\mathrm{mm}$. Their sample spread is zero. In a separate series, the operator removes and reseats the block for every trial, obtaining $19.980,20.000,20.020\,\mathrm{mm}$. That series gives
$$
\bar x=20.000\,\mathrm{mm},\qquad s=\sqrt{\frac{(-0.020)^2+0^2+0.020^2}{2}}=0.020\,\mathrm{mm}.
$$
The two procedures repeat different parts of the experiment. The first data cannot establish the repeatability of the full remove-and-reseat procedure, nor does its zero observed spread prove an absence of readout noise or finite <measurement resolution>.
A repeatability experiment keeps the declared conditions stable and records each completed trial, including placement and settling when those belong to the procedure. <Positioning repeatability> applies this reasoning to repeated returns of a moving stage. Repeating a study over longer periods with selected changes can instead assess <intermediate measurement precision>; studies changing locations, operators and measuring systems concern <measurement reproducibility>.<International vocabulary of metrology by JCGM>[VIM3entries2.22–2.25, changed and unchanged conditions]{ref} Report those changes rather than describing every repeated experiment as the same repeatability test.
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