ISO/TC 28/WG 2 - Determination and application of precision data in relation to methods of test

This document specifies statistical methodology for assessing the expected agreement between two test methods that purport to measure the same property of a material, and for deciding if a simple linear bias correction can further improve the expected agreement. This document is applicable for analytical methods which measure quantitative properties of petroleum or petroleum products resulting from a multi-sample-multi-lab study (MSMLS). These types of studies include but are not limited to interlaboratory studies (ILS) meeting the requirements of ISO 4259-1 or equivalent, and proficiency testing programmes (PTP) meeting the requirements of ISO 4259-3 or equivalent. The methodology specified in this document establishes the limiting value for the difference between two results where each result is obtained by a different operator using different apparatus and two methods X and Y, respectively, on identical material. One of the methods (X or Y) has been appropriately bias-corrected to agree with the other in accordance with this practice. This limit is designated as the between-methods reproducibility. This value is expected to be exceeded with a probability of 5 % under the correct and normal operation of both test methods due to random variation. NOTE Further conditions for application of this methodology are given in 5.1 and 5.2.

This document specifies the process and methodology for the construction, operation, and maintenance of statistical control charts to assess if a laboratory's execution of a standard test method is in-statistical-control and how to establish and validate the 'in-statistical-control' status. It specifies control charts that are most appropriate for ISO/TC 28 test methods where the dominant common cause variation is associated with the long term, multiple operator conditions. The control charts specified for determination of in-statistical-control are: individual (I), moving range of 2 (MR2), and either the exponentially weighted moving average (EWMA) or zone-based run rules [similar to Western Electric (WE) run rules[3]] as sensitivity enhancement strategy to support the I-chart. The procedures in this document have been primarily designed for numerical results obtained from testing of control samples prepared from a homogenous source of petroleum and related products in a manner that preserves the homogeneity of properties of interest between control samples. If the test method permits, a certified reference material (CRM) sample is used as a control sample provided the sample composition is representative of the material being tested and is not a pure compound; if this is done then the laboratory best establishes its own mean for the CRM sample. This document is applicable to properties of interest that are (known to be) stable over time, and for data sets with sufficient resolution to support validation of the assumption that the data distribution can be approximately represented by the normal (Gaussian) model. Mitigating strategies are suggested for situations where the assumption cannot be validated.

This document specifies the methodology for the regular monitoring of the test method precision achieved versus the precision published in the standard test method using data from proficiency testing schemes (PTSs) supported by the regular users of standard test methods. The procedures in this document are designed specifically for proficiency testing (PT) conducted on standard test methods, having a published reproducibility, for petroleum and petroleum-related products, which are presumed to be homogeneous, and where the data distribution is approximately normal. In addition, it is applicable to properties of interest that are (known to be) stable over time and transport. This document specifies the methodology for the statistical comparison of standard deviation under reproducibility conditions achieved in PT programmes versus that published. The purpose of this comparison is to find out if the published reproducibility precision is representative of that achievable by the regular participants in the PT programmes. This document also provides guidance on how to use a PT z-score to monitor an individual participant's performance over time (see Annex B).

ISO 4259-2:2017 specifies the methodology for the application of precision estimates of a test method derived from ISO 4259‑1. In particular, it defines the procedures for setting the property specification limits based upon test method precision where the property is determined using a specific test method, and in determining the specification conformance status when there are conflicting results between supplier and receiver. Other applications of this test method precision are briefly described in principle without the associated procedures. The procedures in ISO 4259-2:2017 have been designed specifically for petroleum and petroleum-related products, which are normally homogeneous. However, the procedures described in ISO 4259-2:2017 can also be applied to other types of homogeneous products. Careful investigations are necessary before applying ISO 4259-2:2017 to products for which the assumption of homogeneity can be questioned.

ISO 4259-1:2017 specifies the methodology for the design of an Interlaboratory Study (ILS) and calculation of precision estimates of a test method specified by the study. In particular, it defines the relevant statistical terms (Clause 3), the procedures to be adopted in the planning of ILS to determine the precision of a test method (Clause 4), and the method of calculating the precision from the results of such a study (Clauses 5 and 6). The procedures in ISO 4259-1:2017 have been designed specifically for petroleum and petroleum related products, which are normally considered as homogeneous. However, the procedures described in ISO 4259-1:2017 can also be applied to other types of homogeneous products. Careful investigations are necessary before applying ISO 4259-1:2017 to products for which the assumption of homogeneity can be questioned.

ISO 4259:2006 covers the calculation of precision estimates and their application to specifications. In particular, it contains definitions of relevant statistical terms, the procedures to be adopted in the planning of an inter-laboratory test programme to determine the precision of a test method, the method of calculating the precision from the results of such a programme, and the procedure to be followed in the interpretation of laboratory results in relation both to precision of the test methods and to the limits laid down in specifications. The procedures in ISO 4259:2006 have been designed specifically for petroleum and petroleum-related products, which are normally homogeneous. However, the procedures described in this International Standard can also be applied to other types of homogeneous products. Careful investigations are necessary before applying ISO 4259:2006 to products for which the assumption of homogeneity can be questioned.

Covers the calculation of precision estimates and their use to specifications. In particular, it contains definitions of relevant statistical terms, the procedures to be adopted in the planning of an inter-laboratory test programme, the method of calculating the precision from the results of such a programme and the procedure to be followed in the interpretation of laboratory results in relation both to precision of the methods and to the limits laid down in the specification.