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. The term "accuracy", used in earlier editions of Practice E 177, 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 -X1

1.3 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 limitations prior to use.