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AAFCO Proficiency Testing Program
               - Statistical Treatment
               - Reports
               - Supporting Articles
Documentation: Statistical procedures and approaches.

Here I have provided a pdf presentation in slide form describing the calculations and parameters used to summarize the information from a sample dataset as well as The International Harmonized Protocol which, along with ISO 13528 (2015) are our guiding documents.

AAFCO Reports AAFCO PTP Statistics Documentation.
AAFCO Reports The International Harmonized Protocol For The Proficiency Testing Of Analytical Chemistry Laboratories (IHP, 2006).

Documentation: Examples

Each of the four Proficiency Testing Schemes is identified on reports by its specific logo as follows:

AAFCO Reports        AAFCO Reports       AAFCO Reports       AAFCO Reports

Here are examples of Report Cards and the Master Lists for all the data derived from a single sample.

AAFCO Reports Example of a Proficiency Test Report Card.
AAFCO Reports Example of an Individual Method Proficiency Test Report Card.
AAFCO Reports Example of a Method Proficiency Testing Report
AAFCO Reports Example of an Analyte Proficiency Testing Report
AAFCO Reports Example of a Method Performance & Summary Report.
AAFCO Reports Example of an Analyte Summary Report.

Documentation: Sample Homogeneity and Stability Evaluation

Here are presentations on sample homogeneity and stability monitoring. Of particular interest is a pseudo homogeneity/stability check we have established for every sample.

AAFCO Reports Homogeneity Monitoring.
AAFCO Reports Stability Monitoring.

Documentation: Mycotoxin Scheme

For the Mycotoxin Scheme samples are carefully prepared by comminution and quantitatively blending a contaminated feed material with a feed matrix of interest. The finished bulk sample undergoes detailed Mycotoxin analysis by a reference lab including Homogeneity testing. It is then split and distributed to clients for their analysis.

When the program started the Assigned value was provided by a reference lab and not based on the consensus of participants. We quickly learned that the assigned value was occasionally quite far from the Consensus Value. At the 2015 mid year meeting in San Antonio it was agreed to use the Consensus Value as calculated using robust statistics. The fit-for-purpose standard deviation is derived from a modified Horwitz relationship. The following pdf presentation describes this statistical approach.

AAFCO Reports Mycotoxin Scheme.

The following presentation describes the January 2015 protocol change to Consensus Value for determining Z scores.

AAFCO Reports Revised Mycotoxin Scheme.

Documentation: Mycotoxin Scheme, 2018 ffp sigma Modification

In reviewing Mycotoxin robust SDs over the last 3 years (2015 to Q1 2018, 13 samples) we find that even the Thompson modification of the Horwitz relationship does not hold true for our 50 clients. The observed robust SDs are severely underestimated by the Thompson modification and somewhat less by the original Horwitz relationship.

What follows is a description of a redesigned Horwitz like data fit for the AAFCO Mycotoxin Scheme.

At the 2018 annual AAFCO Meeting in Fort Lauderdale, the PT committee agreed to make the described changes starting with the 2018 third Quarterly Mycotoxin round.

AAFCO Reports Revised Mycotoxin ffp Sigma, 2018 Decision.

The Following are Examples of Available Reports pre 2020.

AAFCO Reports Example of a Mycotoxin Scheme Proficiency Test Report Card.
AAFCO Reports Mycotoxin Scheme Example of a Proficiency Testing Report.

Documentation: Mycotoxin Scheme, 2020 Revised Reporting.

At the 2020 Mid Year AAFCO meeting in Albuquerque, the PT committee agreed to moving Mycotoxin Scheme reporting to the same format as Animal Feed, Pet Food and Minerals Schemes reporting. We now have enough clients to warrant reporting individual method PT statistics. Also, uniform Scheme reporting provides for more facile accreditation review.

The Following slide presentation describes the new reports in some detail:.

AAFCO Reports New Mycotoxin Reports, January 2020 Presentation

All reports now appear as described in "Documentation: Examples" above. The 2018 revised ffp sigma is still in effect for Mycotoxins as described above.

Documentation: Minerals Scheme

For the Minerals Scheme samples are prepared from prior check sample material. Spike minerals are carefully blended into the base material as described below. The blended batch is then split and distributed to clients for their analysis.

AAFCO Reports Minerals Scheme.

The Following are Examples of Available Reports.

AAFCO Reports Example of a Minerals Scheme Proficiency Test Report Card.
AAFCO Reports Minerals Scheme Example of a Proficiency Testing Report.

Related Documents: published by The Analytical Methods Committee of The Royal Society of Chemistry (AMC)

Here I have provided some selected short supporting documents with emphasis on short. There is a great deal of literature describing the applications of Robust Statistics over an extremely diverse range of subjects. John Tukey, Peter Huber and Frank Hampel have been credited with founding the discipline and all this since the landmark paper by Tukey in 1960. So, you can see that this is a fairly young science nevertheless, the literature and bookshelves are rife with the subject.

AAFCO Reports Analytical Methods Committee 2 page Technical Brief describing the Robust Calculations used in the IHP and ISO 13528.
AAFCO Reports Analytical Methods Committee 2 page Technical Brief on representing data distributions with kernel density estimates.
AAFCO Reports Analytical Methods Committee 2 page Technical Brief describing and using the "Fitness for Purpose" criterion.
AAFCO Reports Analytical Methods Committee 2 page Background Paper "What is proficiency testing?"
AAFCO Reports Analytical Methods Committee 2 page Technical Brief describing Creating your own ffp criteria.
AAFCO Reports Analytical Methods Committee 5 page report on "Robust Statistics - How Not to Reject Outliers", 1989
AAFCO Reports Analytical Methods Committee 2 page Technical Brief on "The amazing Horwitz function"
AAFCO Reports Thompson Modification Of Horwitz Function, "Analyst", vol.125, 2000, p385-386

Interesting Articles:

AAFCO Reports Short 3 page philosophical description of Robust Statistics by Frank Hampel (2001).
AAFCO Reports Short 3 page description of the historical development of Robust Statistics by Elvezio Ronchetti (2006).

Selected Reading: The following are a few pertinent references to which I can not provide open access.

ISO 13528-2015, "Statistical methods for use in proficiency testing by interlaboratory comparison"

"Robust Estimation and Hypothesis Testing", 2nd Ed., Rand R. Wilcox, 2005, Elsevier

"Robust Statistics", 2nd Ed., P.J. Huber and E.M. Ronchetti, 2009, Wiley

"Robust Statistics: The Approach Based on Influence Functions", F.R. Hampel, E.M. Ronchetti, P.J. Rousseeuw and W.A. Stahel, 1986, Wiley

"A survey of sampling from contaminated distributions", J.W. Tukey, (1960). In "Contributions to Probability and Statistics.", I. Olkin, S.G. Ghurye, W. Hoeffding, W.G. Madow and H.B. Mann (Eds), Stanford University Press, pp. 448485.





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Andy

Dr. Andy Crawford, Statistics
andy@crawford.org