1. 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. Note 1-The term "accuracy", used in earlier editions of Practice E177, embraces both precision and bias (see Section 20 and Note 4). 1.2 Informal descriptions of the concepts are introduced in the text as the concepts are developed, and appear in the following sections: Section Terminology 3 Significance and Use 4 GENERAL CONCEPTS Test Method 5 Measurement Terminology 6 Observation 7 Test Determination 8 Test Result 9 SOURCES OF VARIABILITY Experimental Realization of a Test Method 10 Operator 11 Apparatus 12 Environment 13 Sample 14 Time 15 STATISTICAL CONCEPTS Accepted Reference Value 16 Statistical Control 17 Precision 18 Bias 19 Accuracy 20 Variation of Precision and Bias with Material 21 Variation of Precision and Bias with Sources of Variability 22 COMBINATIONS OF SOURCES OF VARIABILITY Repeatability and Laboratory Bias 23 Other Within-a-Single Laboratory Precisions 24 Reproducibility and Bias of the Test Method 25 Range of Materials 26 METHODS OF EXPRESSING PRECISION AND BIAS Indexes of Precision 27 Preferred Indexes of Precision for ASTM Test Methods 28 Preferred Statements of Bias for ASTM Test Methods 29 Elements of a Statement of Precision and Bias 30 STATEMENTS OF PRECISION AND BIAS Examples of Statements of Precision and Bias 31 APPENDIX Alphabetical List of Descriptions of Terms from the Text XI 1.3 This standard does not purport to address the safety problems 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 limitations prior to use.