4.1 To describe the performance of a standard test method, precision and bias statements are required.5 Committee E11 on Probability and Statistics has published standards addressing these terms and the preparation of precision and bias statements; these include Practices E177 and E691. These practices should not be used in cases where small numbers of test results do not support statistical normality.

4.2 The intent of this guide is to help analysts prepare and interpret precision and bias statements. It is essential that, when the terms are used, their meaning should be clear and easily understood.

4.3 Appendix X1 and Appendix X2 provide the theoretical foundation for precision and bias concepts and Practice E691 addresses the problem of sources of variability. To illustrate the interplay between sources of variability and formulation of precision and bias statements, a hypothetical data set is analyzed in Appendix X2. This example shows that depending on how the data was collected, different precision and bias statements are possible. Reference to this example will be found throughout this guide.

4.4 There has been much debate inside and outside the statistical and metrological community on the exact meaning of some statistical terms. Thus, following a number of the terms in Section 3 is a list of several ways in which that term has been used. This listing is not meant to indicate that these meanings are equivalent or equally acceptable. The purpose here is more to encourage clear definition of terms used than to take sides. When these terms are used, they should be defined clearly.

4.5 This guide is intended as an aid to understanding the statistical concepts used in precision and bias statements. There is no intention that this be a self-contained introduction to statistics. Since many analysts have no formal statistical training, it is advised that a trained statistician be consulted for further clarification if necessary.

1.1 This guide covers terminology useful for the preparation and interpretation of precision and bias statements. This guide does not recommend a specific statistical method. It provides awareness of terminology and approaches and options to use for precision and bias statements.

1.2 In formulating precision and bias statements, it is important to understand the statistical concepts involved and to identify the major sources of variability that affect results. Appendix X1 and Appendix X2 provide a brief summary of these concepts.

1.3 To illustrate the statistical concepts and to demonstrate some sources of variability, a hypothetical data set has been analyzed in Appendix X2. Reference to this example is made throughout this guide.

1.4 It can be difficult to ship nuclear materials for interlaboratory testing. When this is the case, precision statements may reflect only within-laboratory variability.

1.5 No units are used in this statistical analysis.

1.6 This guide does not involve the use of materials, operations, or equipment and does not address any risk associated.

1.7 This international standard was developed in accordance with internationally recognized principles on standardization established in the Decision on Principles for the Development of International Standards, Guides and Recommendations issued by the World Trade Organization Technical Barriers to Trade (TBT) Committee.