This International Standard specifies the methodology for the regular monitoring of the test method precision achieved versus precision published in the standard test method using data from Proficiency Testing (PT) schemes supported by the regular users of standard test methods. The procedures in this International Standard are designed specifically for PTs' conducted on standard test methods for petroleum and petroleum related products, which are presumed to be homogeneous and where the data distribution is approximately normal. The procedures in this document are designed specifically for standard test methods with published reproducibility derived from ISO 4259-1 or equivalent (such as ASTM D6300[1]) for petroleum and petroleum related products, which are normally considered as homogeneous. In particular, this document specifies the methodology for the statistical comparison of standard deviation under reproducibility conditions achieved in PT programmes versus that published. Purpose of this comparison is to ascertain if the published reproducibility precision is representative of that achievable by the regular participants in the PT programmes. Guidance on how to use PT zz score to monitor an individual participant's performance over time is provided as an Informative Annex.