21/30384248 DC

This standard 21/30384248 DC BS EN ISO 4259-4. Petroleum and related products. Precision of measurement methods and results is classified in these ICS categories:

• 75.080 Petroleum products in general

This document specifies the work process and methodology for the construction, operation, and maintenance of statistical control charts for the purpose of assessing if a laboratory's execution of a standard test method is in statistical control. By using statistical control charts and following this document the 'in-statistical-control' status is established and validated.

This document explicitly defines ‘site precision’ conditions as single apparatus, multi-operators, over a long time horizon. 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 as described by “site precision” 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) 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.

This document applies 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 aforementioned assumption cannot be validated.