Here are some formal definitions. Precision The precision of a set of measurement values refers to how closely the values agree with one other, independently of how close to or far from the "true value" they may be. Accuracy The accuracy of a set of measurement values describes how close the values of the set are to the "true value".

A difficulty in using the definition of accuracy is that one does not know in general what the "true value" is when all one has is results of individual measurements.