trueness

Trueness is a term used in metrology to describe how close the average of a large set of measurements is to the true or reference value. It is a measure of the systematic error, or bias, of a measurement method and is distinct from the random variation observed in individual measurements.

Unlike the ordinary sense of accuracy, which is sometimes used to describe a single result, trueness refers to the long-run closeness of the method's mean result to the true value. A measurement method can have high trueness but low precision if repeated results vary widely, and vice versa.

Trueness is assessed by repeated measurements against a known standard or certified reference value. The mean of many replicated results is compared with the reference value; the difference is the trueness error or bias. The use of certified reference materials helps quantify trueness.

Standards in measurement science, such as ISO 5725, define trueness as the closeness of agreement between the mean of a large number of results and a reference value. In practice, the mean of a finite set is used as an estimate of trueness, with uncertainty attributed to sampling.

Outside metrology, "trueness" can mean fidelity to truth or factual accuracy in a broader sense, but in scientific contexts it is primarily the property of a measurement method relating to bias.