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Precision and accuracy

Precision and accuracy are terms widely used in the world of measurements, being generally used to qualify the methods of measurement or even measuring instruments, form indicate your performance. The problem is that the formal meaning of these terms is not very well known to the public, metrology and often are confused with each other, and with other completely different concepts.  This not to mention the translations and versions of these terms in other languages.  It is useful, in this way, consult the references more solid looking for more correct settings.

On the brazilian version of the International Vocabulary of metrology, CAME the 2012 following settings:

Accuracy or accuracy of measurement, or em English , accuracy is the closeness of agreement between a measured value and a value (true) of a measurand.  The term is qualitative, means you do not associate the concept with numbers.

Precision of measurement, sometimes called fidelity for measurement, in English precision , is the degree of agreement between indications or measured values obtained by repeated measurements on the same object or similar objects under specified conditions.  The accuracy can be expressed numerically in terms of standard deviations, variances or coefficients of variation.

In addition to these, we also have the term Accuracy of measurement truenessin English, is the closeness of agreement between the average of an infinite number of repeated measurements and a reference value. The truth is also a qualitative term, is not expressed numerically.

In practice, the accuracy is a characteristic linked to random errors of a measurement method, the veracity is linked to systematic errors of the method, and the accuracy is actually tied both the random errors as the systematic errors. Some people still use the term accuracy, reminding the English term accuracy to represent the accuracy of a method.

In practice, these three characteristics need to be estimated numerically, for this we use quantitative expressions that relate to them.

Tipo de erroCaracterísticaEstimativa numérica
Erros sistemáticosVeracidade / TruenessTendência / Bias
Erros AleatóriosPrecisão / precisionDesvio padrão
Erros totaisExatidão / accuracyIncerteza de medição

I recommend reading the article of Menditto (2007) which features an excellent analysis of these terms.

References:

International vocabulary of Metrology: fundamental concepts and General and associated terms (VIM 2012). JCGM 200.2012.3rd, translated by INMETRO, 2012, 94 p.

Menditto, Antonio & Patriarch, Marina & Magnusson, Bertil. (2007). Understanding the meaning of accuracy, trueness and precision. Accreditation and Quality Assurance. 12.45-47. 10.1007/s00769-006-0191-z.