ISO5725-1 Introduces Confusion

For engineers who have been taught that accuracy is the "nearness of a measurement to the true value" and that precision is the "degree to which several measurements are close to each other," the introduction of these definitions may cause come confusion.

Specifically, the new definition of accuracy refers to both trueness and precision, while the traditional definition explicitly differentiates accuracy as separate from precision.

Arguably, the newer ISO definition is a more natural way of looking at accuracy, however this definition is not widely used in the USA, yet. Therefore, it is important that you understand the definitions of these terms as you create, read, or work with specifications that use them.

The big clue is the inclusion of the definition for trueness. If the word is defined, it is very likely that you are dealing with a specification that adheres to the ISO 5725-1 definition.

In 1994 ISO introduced their standard 5725, Accuracy (trueness and precision) of measurement methods and results.

The stated purpose of this standard is: [T]o outline the general principles to be understood when assessing accuracy (trueness and precision) of measurement methods and results, and in applications, and to establish practical estimations of the various measures by experiment.

[The standard] [i]s concerned exclusively with measurement methods which yield measurements on a continuous scale and give a single value as the test result.

[The standard] [m]ay be applied to a very wide range of materials, including liquids, powders and solid objects, manufactured or naturally occurring, provided that due consideration is given to any heterogeneity of the material.

It introduced the following definitions:

Accuracy:
1) The term "accuracy," when applied to a set of test or measurement results, involves a combination of random components and a common systemic error or bias component.
2) Accuracy refers to a combination of trueness and precision.
The closeness of agreement between a test result and the accepted reference value (i.e., the "true" value).
Trueness:
1) The measurement of trueness is typically expressed in terms of bias.
2) Trueness is sometimes referred to as "accuracy of the mean." This usage is not recommended by ISO 15725-1 (draft).
The closeness of agreement between the average value obtained from a large series of test results and an accepted reference value.
Precision:
1) Precision depends only on the distribution of random error. It is not related to the true value.
2) The measure of precision is usualy expressed in terms of imprecision and is stated as a standard deviation of measured results.
The closeness of agreement between independent test results obtained under stipulated conditions.