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


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