Significance and Use
5.1 This guide presents techniques and guidance for evaluating and assuring homogeneity of individual samples or bulk materials and can be used for either interlaboratory or intra-laboratory studies. The types of studies include, but are not limited to, studies to determine precision estimates for test methods, proficiency testing programs, and studies related to quality control of testing within a single laboratory.
5.2 Because the test results of any laboratory study are affected by the quality of the samples tested, producing homogeneous samples and determining the degree of homogeneity is important for interpreting the results of the study.
5.3 Five techniques are presented in this guide to evaluate sample homogeneity for a range of circumstances and degrees of rigor. The circumstances under which the studies are conducted and the degree of rigor required may differ. The user should consider the circumstances listed in each technique to determine which is appropriate for the study at hand.
5.4 Each of the Techniques 1, 2, and 3 provides a procedure for testing and evaluating sample homogeneity when replicate testing of the samples is possible. Technique 4 provides a plan to evaluate sample homogeneity when replicate testing is not possible. Technique 5 recommends practices for producing homogeneous samples for circumstances when homogeneity testing is not possible.
5.5 When the conditions of adequate within-sample homogeneity and between-sample homogeneity are satisfied, any differences in test results on multiple samples can reasonably be attributed to testing variation and not due to sample variation.
5.6 When differences within or between samples are discovered and the samples are deemed insufficiently homogeneous, the sample preparation process can be improved or corrected and a new set of samples can be prepared. Or, in cases where the sample homogeneity cannot be improved or for other reasons when the samples must be used, the method of evaluation for the laboratory study should account for the effect of differences between samples.
5.7 When used in conjunction with studies to develop precision estimates, the guidance in this standard can be used to help quantify sources of test variation (such as effects due to sampling, test method repeatability, and the degree of inhomogeneity) and, therefore, can be useful for determining and stating the conditions under which the precision estimates are valid.
5.8 For proficiency testing programs, the guidance in this standard can provide information to prevent laboratories from being unfairly penalized for testing variation due to inherent differences between samples.
5.9 In a single laboratory, the guidance in this standard could be used to evaluate the homogeneity of samples for studies to measure test variation over time or for studies to compare the results of tests performed by different technicians.
5.10 To minimize the resources required for homogeneity testing, a testing design using a minimum of ten samples with two replicate tests performed on each sample is recommended in Techniques 1, 2, and 3 of this guide. This test design is used in other international standards. See Ref (1)4 and ISO 13528. Technique 4, used when replicate testing is not possible, similarly recommends testing a minimum of ten samples. That does not preclude the use of more than ten samples or more than two replicates.
NOTE 1: The spreadsheets provided in this guide for the examples in Techniques 1, 2, and 3 show the calculations when two replicate tests are performed on each sample. The spreadsheets shown for Techniques 1 and 2 may be adjusted using the equations provided in the text when more than two replicate tests are used. Use of Technique 3, as presented in Section 9, is limited to duplicate testing (that is, k = 2). To use Technique 3 when k > 2, preliminary testing for consistency of replicate results can be performed using the general form of the Cochran’s Test as presented in Technique 1, and the homogeneity analysis can be performed as described in the Appendix, X4.3. Also, if desired, the homogeneity criterion in Technique 3 can be used with the calculations using the spreadsheets shown in Technique 2.
5.11 This guide is not sufficient for evaluation of certified reference materials (CRMs) or materials used for calibration. Even though homogeneity is required for CRMs, CRMs and calibration materials are typically subject to additional requirements (such as traceability and estimates of uncertainty) that are not addressed in this guide.
Scope
1.1 This guide presents techniques and guidance for evaluating and assuring homogeneity of individual samples and bulk materials used for interlaboratory and intra-laboratory studies.
1.2 This guide is applicable to samples and reference materials used for proficiency testing programs and for interlaboratory studies to determine precision estimates for test methods. It may also be useful for activities related to quality control of testing within a single laboratory.
1.3 Five techniques are presented for assessing sample homogeneity. The five techniques are not an exhaustive list of available techniques for assessing homogeneity of samples, but the techniques were chosen to cover a range of circumstances (and various degrees of rigor required) for laboratory studies of various types and purposes.
1.4 Each of the first four techniques provides a scheme for testing for homogeneity and a statistical procedure for evaluating the results of the homogeneity testing. The circumstances are described for which each of the techniques is suited.
1.5 For circumstances when homogeneity testing is not possible, the fifth technique provides guidance for producing homogeneous samples.
1.6 The appendixes of this guide provide example spreadsheets for Techniques 1, 2, 3, and 4.
1.7 This guide is not intended for evaluation of certified reference materials (CRMs) or materials used for calibration.
1.8 Units—The system of units for this standard is not specified. Dimensional quantities in the standard are presented only as illustrations of calculation methods. The examples are not binding on products or test methods treated.
1.9 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, health, and environmental practices and determine the applicability of regulatory limitations prior to use.
1.10 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.
