Proficiency testing (PT) is together with QC an essential element of laboratory quality assurance. Regular proficiency testing provides independent feedback on the quality of analytical results, enabling laboratories to monitor and improve performance over time. It is also the most important test if the estimated measurement uncertainty by the lab is correct. In many cases we underestimate the uncertainty...
Eurachem provides detailed guidance on PT and in a Eurachem task view all information available on the website is put together.
The EPTIS database helps you find a suitable proficiency testing (PT) scheme for your laboratory
For the evalaution of your PT results the Trollbook says
Evaluation of proficiency tests
If the laboratory regularly participates in proficiency tests of similar nature, plotting the PT results in control charts (similar to an X-chart) provides the quality manager with a good overview over the performance, including possible systematic effects or trends. See further chapter 11. Note the choice between z-score and zeta score