Proficiency Test Interpretation

I have joined a seminar entitled “How to Interpret Performance in Proficiency Test / External Quality Assessment Scheme” organized by Hong Kong Accreditation Service (HKAS) on 11 Sep 2009. The speaker was Mr. Daniel Tholen (convenor of the ISO drafting committee on ISO/IEC 17043; and the chair of the APLAC Proficiency Testing Committee). This seminar reviewed the accreditation requirements of HKAS, APLAC (Asia Pacific Laboratory Accreditation Cooperation) and ILAC (International Laboratory Accreditation Cooperation) and summarized as follows.

What is proficiency testing? It is an evaluation of participant performance against pre-established criteria by means of interlaboratory comparisons.


In the beginning, Mr. Tholen introduced the responsibility of Proficiency Test (PT) provider, Laboratory and Accreditation Body. PT provider should ensure their competence and quality to comply with the accreditation requirements; it should establish open communication policy, document both procedures and complaint, as well as, maintain confidentiality. Accreditation Body should use ISO/IEC 17043 to assist design of appropriate PT, establish policy and train assessors on proper use of PT. Laboratories should choose competent PT provider and they need to follow the instructions from the coordinator and the accreditation body. The most important is to use routine method for PT and self-improvement.

He also briefed the ILAC P9 requirements and APLAC PT 006 Benchmarks. The frequencies for different areas of testing and calibration were shown as follows:
· Medical: 10/yr
· Chemical (environmental and mineral): 2/yr
· Biological: 2/yr
· Forensics, chemical, veterinary: 1/yr
· Specific Calibration fields: 1 per 2 years

Apart from the requirements of ILAC P9, the accreditation body should ensure participation when PT is available, appropriate and at a frequency that reflects best practices and/or best local norms.

Based on ISO/IEC FDIS 17043, the Varieties of Proficiency Testing included:
Sequential measurement
Simultaneous (bulk) testing
Single time programs
Continuous programs
Measurable (quantitative) properties
Nominal and ordinal properties
Data transformation or interpretation
Inspection and sampling

Mr. Tholen mentioned the preventive actions, investigating non-conformance, root cause analysis, corrective actions and monitoring of corrective actions. During monitoring the corrective actions, it could be found that correction might not be correct! Lastly, he discussed the policy on confidentiality. There are four questions for discussion below:
· Strictly confidential?
· Shared with accreditation body?
· Shared with regulatory agency?
· Shared with other participants?


The next section was interpretation of the PT results. Mr. Tholen showed several PT reports regarding Total Heavy Metal, Melamine in Food and in Fish, and DC current calibration.

Z-score is very common in use for PT. z = (X – X*) / SD where X* is computed mean from robust statistics and SD is standard deviation of estimated from robust statistics.
Interpretation: Z ≦ 2 (Satisfactory); 2 < Z < 3 (Questionable); Z ≧ 3 (Unsatisfactory).


Reference:
Presentation file at http://www.itc.gov.hk/en/quality/hkas/doc/seminars/Seminar%20Materials/ProficiencyTesting/interpretperformance.ppt

APLAC PT 006:2008 - Proficiency Testing Frequency Benchmarks

ILAC-P9:2005 – ILAC Policy for Participation in National and International Proficiency Testing Activities

ISO/IEC FDIS 17043 – Conformity assessment – General requirements for proficiency testing

ISO 13528:2005 – Statistical methods for use in proficiency testing by interlaboratory comparisons