4.1 Part A of the "Blue Book," Form and Style for ASTM Standards, requires that all test methods include statements of precision and bias. This practice discusses these two concepts and provides guidance for their use in statements about test methods.

4.2 Precision-A statement of precision allows potential users of a test method to assess in general terms the test method's usefulness with respect to variability in proposed applications. A statement of precision is not intended to exhibit values that can be exactly duplicated in every user's laboratory. Instead, the statement provides guidelines as to the magnitude of variability that can be expected between test results when the method is used in one, or in two or more, reasonably competent laboratories. For a discussion of precision, see 8.1.

4.3 Bias-A statement of bias furnishes guidelines on the relationship between a set of typical test results produced by the test method under specific test conditions and a related set of accepted reference values (see 9.1).

4.3.1 An alternative term for bias is trueness, which has a positive connotation, in that greater bias is associated with less favorable trueness. Trueness is the systematic component of accuracy.

4.4 Accuracy-The term "accuracy," used in earlier editions of Practice E177, embraces both precision and bias (see 9.3).

The purpose of this practice is to present concepts necessary to the understanding of the terms "precision" and "bias" as used in quantitative test methods. This practice also describes methods of expressing precision and bias and, in a final section, gives examples of how statements on precision and bias may be written for ASTM test methods. A statement of precision allows potential users of a test method to assess in general terms the test method's usefulness with respect to variability in proposed applications.

In this practice only quantitative test methods that produce numerical results are considered. Also, the word "material" is used to mean material, product, system or service; the word "property" is used herein to mean that a quantitative test result can be obtained that describes a characteristic or a quality, or some other aspect of the material; and "test method" refers to both the document and the procedure described therein for obtaining a quantitative test result for one property.