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

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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

**************************************************************************************************FAQ for proficiency testing (PT)
Here are some common questions and answers on PT

  1. How to select a criteria for the PT based on you own uncertainty?
    It is better if your standard uncertainty is less than or equals to the standard deviation, s used by the PT provider for calculating the z-score. Example with pH for a water laboratory measuring pH in natural water (lake, river…) where the conductivity is low and therefore it is more difficult to measure pH. The measurement uncertainty, U = 0.2 pH units and the standard uncertainty is U/2 = 0.1 pH.

    The s for calculating z-score is for this PT provider 0.1 pH units. If for one PT sample analysed by the lab the difference from the assigned value is 0.2 pH unit this will give a z-score of 2 and is also at the limit of the expanded measurement uncertainty. This would be an accepted performance.

    If however the laboratory had estimated the U = 0.15 pH the z-score will still be 2 which could be regarded as OK, but a difference of 0.2 pH units will be outside the uncertainty given by the laboratory.
    Solution: Calculate in this case zeta score - see Trollbok and below.

  2. Use of zeta score to evaluate your PT results.
    Here you need:
    1) standar (combined) uncertainty of you lab, u(lab) equal to U/2; and
    2) standard uncertainty of the assigned value. This can be given by the PT provider or calculated as s(R)/sqrt(n_lab). This will indicate if the U you have reported is valid. For the formula for zeta score see Trollboken,