Significance and Use
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.
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 Section 15.
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 Section 16).
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.
Accuracy—The term “accuracy,” used in earlier editions of Practice E 177, embraces both precision and bias (see Section 17 and Note 3).
A Table of Contents is shown below, listing the concepts in this standard.
| TABLE OF CONTENTS | Section |
| GENERAL CONCEPTS | |
| Test Method | 5 |
| Measurement Terminology | 6 |
| SOURCES OF VARIABILITY | |
| Experimental Realization of a Test Method | 7 |
| Operator | 8 |
| Apparatus | 9 |
| Environment | 10 |
| Sample | 11 |
| Time | 12 |
| STATISTICAL CONCEPTS | |
| Accepted Reference Value | 13 |
| Statistical Control | 14 |
| Precision | 15 |
| Bias | 16 |
| Accuracy | 17 |
| Variation of Precision and Bias with Material | 18 |
| Variation of Precision and Bias with Sources of Variability | 19 |
| COMBINATIONS OF SOURCES OF VARIABILITY | |
| Repeatability and Laboratory Bias | 20 |
| Other Within-a-Single Laboratory Precisions | 21 |
| Reproducibility and Bias of the Test Method | 22 |
| Range of Materials | 23 |
| METHODS OF EXPRESSING PRECISION AND BIAS | |
| Indexes of Precision | 24 |
| Preferred Indexes of Precision for ASTM Test Methods | 25 |
| Preferred Statements of Bias for ASTM Test Methods | 26 |
| Elements of a Statement of Precision and Bias | 27 |
| STATEMENTS OF PRECISION AND BIAS | |
| Examples of Statements of Precision and Bias | 28 |
Scope
1.1 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.
1.2 This standard does not purport to address all of the safety concerns, if any, associated with its use. It is the responsibility of the user of this standard to establish appropriate safety and health practices and determine the applicability of regulatory requirements prior to use.
