Approaches to establishing the assigned value

There are, as described in ISO 13528, essentially five methods available to obtain the assigned value, a working estimate of the true value:

• Formulation; the addition of a known amount or concentration of analyte to a base material containing none.

   

• Certified reference values: when the PT test item is a certified reference material (CRM).

   

• Reference values: a selection of the prepared proficiency test items is tested, by a chosen laboratory, either using a primary method or alongside a certified reference material (CRM). The assigned value is derived directly from the primary method used or from a calibration against the certified reference value of the CRM.

   

• Consensus values from expert laboratories: the determination of a consensus value obtained from the outcome of a group of expert or referee laboratories proficient in the analytical methods applied.

   

• Consensus value from participants; the use of a consensus value produced in each round of the PT and based on the results obtained by the participants.