Frequently Asked Questions


How many participants are typically involved in each test?
This varies by matrix and method. Refer to past reports for past participation.
Is partial involvement on a PT scheme/round acceptable?
Yes – The customer decides the frequency and the analytes reported.
Are partial year subscriptions available at a reduced cost?
No. All subscriptions are offered on an annual basis only. However, customers may enroll midyear. For customers enrolling midyear, past test items from the current year will be shipped with the first shipment, as available. No guarantee is made that all test items will be available since they are produced consistent with enrollment at the time of production.


How are Proficiency Testing materials shipped?
The shipping method is selected by the customer at the point of enrollment. The options include:
  • USPS First Class - USA
  • USPS First Class International parcel service.
  • USPS Express Mail International.
  • International customers only can also use a courier service (must provide account #).
What is expected shipping time?
This varies with the shipping method chosen. For example, with USPS Express Mail estimated shipping time of 3 to 5 business days and full tracking. AAFCO cannot guarantee nor estimate delivery times for international laboratories.
Would there be any shipping delays in customs for international shipments?
There may be issues with customs, this varies by country.
My lab is located outside of the US. Do you provide import/export permits or certificates?

No. It is the responsibility of the customer to secure all needed import permits or certificates.

Homogeneity and Stability

Is a Homogeneity study performed? If so can results be provided?
Homogeneity studies are performed periodically. In lieu of doing studies for each round, markers are monitored statistically.
What assurance is provided for the stability of the proficiency material prior to receipt?
Packaged materials are stable within the reporting time frame under ambient conditions.


What is the minimum number of participants required to apply statistics?
Six Laboratories
What statistical analysis is provided on each PT material? (Consensus vs. mean, standard deviation and median value)
Consensus values, Robust Standard Deviation and uncertainties, Z-Scores and method precision.
What process is used to exclude outliers?
The robust procedures significantly down weights the outliers.
I was looking at the results for the Total Fumonisin and I have not been able reproduce the Z-scores given for our laboratory. Would you please provide a breakdown of how they are calculated?
Z = (Labavg – Robust Mean)/Horwitz σffp
For example:
Labavg = Laboratory Average = (Result1 + Result2)/2 = (3600 + 3700)/2 = 3650
Robust Mean = 2512
Horwitz σffp = Horwitz predicted fit-for-purpose (ffp) standard deviation (σ) = 349.8
Z = (3650 – 2512)/349.8 = +3.25
I notice that the assigned value for the total is not the same as what I would expect if I added the assigned values for FB1, FB2, and FB3 together.
The assigned value for Total Fumonisin is calculated from the values submitted for Total Fumonisins not the individual Fumonisins.
How are the statistics handled for AAFCO PTP?
Robust statistics are used and described at


What is the timeline for delivery of results?
Study reports are available on the last day of the month.
Does the PT provider request and report the different methods used to obtain the results?
Does the PT provider give a certification for approved chemist for the test?

Quality Reference Materials (QRM)

When does a PTP material become available as a QRM?
The material is added to the QRM once the report is released.
AAFCO Headquarters Office
1800 S. Oak Street, Suite 100, Champaign, IL 61820-6974
Phone: 217-356-4221 --- Fax: 217-398-4119
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