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.