There are two measurements that serve to express precision in the evaluation of a standard test method. They are commonly referred to as "repeatability" and "reproducibility" and provide the boundaries between which precision exists.
Bias, on the other hand, is defined as a systematic error that contributes to the difference between the mean of a large number of test results and an accepted reference value. When included in a standard test method, this statement describes the bias and the methods utilized to provide corrected test results. It is important to remember that if an accepted reference value is not available, then the bias cannot be established. However, if the bias is unknown but the direction or bounds of the bias can be estimated, this information should be included in the bias statement.
As precision and bias are mandatory sections of an ASTM standard test method (per Section A.21 Form and Style for ASTM Standards), the utmost care should be taken to ensure that the final data, as well as the steps that were taken to generate the data, are as precise and accurate as possible. A standard test method that is incapable of doing what it purports can be misleading. Precision and bias statements strengthen the perceived validity of the standard test method and provide the user of the document with the added confidence of knowing that the standard test method has been laboratory tested.