ASTM International - ASTM E177-08

Standard Practice for Use of the Terms Precision and Bias in ASTM Test Methods

inactive

Organization:	ASTM International
Publication Date:	1 October 2008
Status:	inactive
Page Count:	12
ICS Code (Test conditions and procedures in general):	19.020

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... View More

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

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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.