Contrary to most other external quality assessment schemes, participating laboratories in IMEP receive the IMEP-certified test sample with undisclosed certified reference values as measured by reference laboratories. These reference laboratories prove their claims of measurement capabilities by means of participation in internationally mutually recognized key comparisons. Certified reference values are required to demonstrate traceability and they should have a demonstrated and adequately small uncertainty, as evaluated according to international guidelines. Hence, these certified reference values are completely independent from the IMEP participants’ result.
Participants of IMEP carry out chemical measurements on these samples in the same way as they would usually do using their routine analytical set-up. By comparing the individual measurement results of the participants with these certified reference values, field laboratories can demonstrate the traceabilty of their measurement results.
Another unique feature of IMEP is that participants in IMEP have always been invited to state uncertainty estimates for their reported results. A large number of laboratories participating in IMEP have to comply with the ISO/IEC 17025 standard (general requirements for the competence of testing and calibration laboratories). They need to meet the requirement of providing reliable measurement results within stated uncertainties.
From this view the continuous expansion of the concepts of IMEP brings together measurement results from different geographical locations, originating from participants using various analytical methods with different metrological function and experience. By means of measurements performed on the same samples, it is possible to see evidence of measurement capability present at various levels of the measurement chain.