The Danish external quality assessment scheme.

Participation in proficiency testing schemes is generally recognized as an important part of quality control in analytical chemistry. The design and conduct of proficiency testing schemes is therefore a matter of great importance. A proficiency testing scheme should: a) cover a broad concentration range of the substance of interest; b) enable the detection and estimation of each laboratory's systematic errors; c) enable estimation of each laboratory's random error; d) provide each laboratory with a rank or score, and e) present the evaluation of each round to the participating laboratories in a comprehensible way, e.g. in a report with carefully designed text, tables and figures. This paper presents the design of the Danish external quality assessment scheme (DEQAS), which is based on the method evaluation function concept.