= Measurement precision
<Measurement precision> describes agreement among repeated indications or measured values obtained under stated conditions. It is commonly characterized using a <standard deviation>, <variance> or <coefficient of variation>, which describe the observed spread. <Measurement repeatability> is precision under a particular restricted set of conditions; <intermediate measurement precision> and <measurement reproducibility> concern other specified conditions.<International vocabulary of metrology by JCGM>[VIM3entry2.15, definition and notes1–3]{ref}
For an original <balance> example, take three repeat readings of a reference object: $100.100$, $100.080$ and $100.120\,\mathrm g$. Their <arithmetic mean> and <sample standard deviation> are
$$
\bar x=100.100\,\mathrm g,\qquad s=\sqrt{\frac{0^2+(-0.020)^2+0.020^2}{3-1}}=0.020\,\mathrm g.
$$
Suppose the reference object's assigned mass is $100.000\,\mathrm g$ with <standard uncertainty> $0.002\,\mathrm g$. The observed mean differs from that reference by $0.100\,\mathrm g$, much more than the within-series scatter. These chosen readings therefore illustrate why close repeat readings do not establish agreement with a reference. They do not determine the true systematic offset from only three trials.
<Measurement accuracy> and <measurement trueness> answer different questions from precision. A common additive offset leaves the deviations $x_j-\bar x$, and hence $s$, unchanged. Averaging more independent readings can reduce random uncertainty in the mean but does not by itself remove that common offset.<International vocabulary of metrology by JCGM>[VIM3entry2.15 note4; entry2.14, trueness and systematic error]{ref}
A useful physical study repeatedly removes and replaces the reference object while recording load procedure, environmental conditions and elapsed time. Specify which parts of the measurement were repeated: repeated display readings alone may omit placement variation or <instrumental drift>. The statistic describes the stated experiment, not an unconditional property of the instrument.
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