The assigned value of PT program
A critical step in the organization of a proficiency testing (PT) scheme is specifying the assigned value for the participating laboratories. The purpose is to compare the deviation of participant’s reported results from the assigned value (i.e. measurement error) with a statistical scoring criterion which is used to decide whether or not the deviation represents significant cause for concern in its performance.
ISO 13528:2015 defines assigned value as “value attributed to a particular property of a proficiency test item”. It is a value attributed to a particular quantity being measured. Such an assigned value will have a suitably small uncertainty which is appropriate for this interlaboratory comparison purpose.
Where do we obtain an assigned value?
A. Assigned value obtained by formulation
A specified known level or concentration of the target analyte is added accurately to a base material preferably containing no native analyte. The assigned value is then derived by calculating the analyte concentration from the masses of analyte used. By this way, the traceability of the assigned value can usually be established.
However, there may be no suitable base material (blank material) or well characterized base material available. Ensuring homogeneity in the prepared bulk material before distributing to the participants may also be a challenge. Furthermore, formulated samples may not be truly representative of test materials as the analyte may be in a different form from the less strongly bound to the matrix.
B. Assigned value is a certified reference value
In this case, the test material is a certified reference material (CRM) made by a reputable organization and the assigned value is therefore the certified value and its uncertainty are quoted on the CRM certificate. The limitations of using this assigned value are:
- Generally the CRM can be expensive to provide every participant with a unit of such CRM
- It is important to conceal the identity of commercial CRM from the participants as the testing outcome may be compromised by the participants
- Some certified value uncertainty may be high.
C. Assigned value is a reference value
The assigned value is determined by a single expert laboratory using a suitable primary method of analysis (e.g., gravimetry, titrimetry, isotope dilution mass spectrometry, etc.) or a fully validated test method which has been calibrated with a closely matched CRM.
D. Assigned value from consensus of expert laboratories
This assigned value is obtained from the results reported by a number of expert laboratories, with demonstrated proficiency in the measurements of interest, which analyze the material using suitable methods. However, it must be cautioned that there may be an unknown bias in the results produced by the expert laboratories.
E. Assigned value from consensus of PT scheme participants
This is the result from all the participants in the proficiency testing round. It is normally based on a robust estimate to minimize the effect of extreme values in the data set.
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