The concept of *Accuracy*, which consists of *Trueness* and *Precision*, is illustrated in the figure

ACCURACY: closeness of agreement between a measured quantity value and a true quantity value of a measurand (2.13 VIM) |

Accuracy is thus determined as a combination of precision and trueness.

PRECISION: closeness of agreement between indications or measured quantity values obtained by replicate measurements on the same or similar objects under specified conditions (2.15 VIM) |

Precision is a qualitative concept without a numerical value. Precision can be expressed numerically with its opposite, for example a standard deviation or variation coefficient (standard deviation divided by the mean value).

TRUENESS: closeness of agreement between the average of an infinite number of replicate measured quantity values and a reference quantity value (2.14 VIM) |

Trueness is expressed numerically in terms of its opposite, namely the systematic error or bias.

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