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.