Trueness represents the difference (called bias) between the measured value and the expected value (true value or reference value).

Trueness is not related to random error: it is a systematic error(s). Therefore, it is not easy to evaluate.

Several approaches can be considered to evaluate trueness. In particular, there is the use of known additions to the sample (known as the recovery of substances added to "spiked" samples) and the use of certified reference materials. Participating in inter-laboratory tests can also help to detect potential systematic errors attributable to the laboratory (but not to the measurement method).