This standard specifies requirements on carrying out interlaboratory trials for external quality control of laboratories. With respect to the purpose the standard differs from ISO/TS 7013 and DIN ISO 5725-2, which are used for quantitative statements about reliability of analytical methods. This document specifies in particular the requirements for the performance of interlaboratory comparisons for the proficiency testing of laboratories in water, wastewater and sludge analysis, insofar as they go beyond the requirements of DIN EN ISO/IEC 17043. The requirements of DIN ISO 13528 concerning the applicable statistics are specified for the scope of this document. This document may be applied if the parameters to be determined in the interlaboratory comparison can be regarded as at least approximately continuously quantifiable. This is usually the case for chemical, physical and physico-chemical parameters, among others.