This International Standard specifies criteria and procedures for comparing the average quantitative results obtained by two microbiological analytical methods, one of which may, but need not, be a standard or reference method. The methods considered are based on counts of colonies or of positive and negative liquid enrichment tubes (MPN, most probable number). It specifies an evaluation procedure for comparing two methods intended for the detection or quantification of the same target group or species of microorganisms. This International Standard provides the mathematical basis for the evaluation of the average relative performance of two quantitative methods against chosen criteria for the comparison. The document does not provide data for assessment of the precision of the methods being compared. The precision of methods should be assessed as part of their performance characterization. This International Standard does not provide methods for the verification of method performance characterization in a single laboratory. The decision rules are described according to these two methods are considered as "not different". The most important basic requirement of comparison studies is a wide range of samples. Participation by a number of laboratories is usually necessary, allowing the expansion of the sample range over large geographical areas. Also the credibility of a general conclusion is commonly believed to require the participation of several laboratories. The result of the comparison is generally valid only within the range of sample types studied. Advice on the conduct of comparison studies is given. It is important to note that the principles of operation of the two methods being compared should be well understood and that the significance of any differences in the methods on the outcome of the comparative assessment should be recognized. This is particularly important if the confirmed results from each method are based on different principles. This document (EN ISO 17994:2014) has been prepared by Technical Committee ISO/TC 147 "Water quality" in collaboration with Technical Committee CEN/TC 230 "Water analysis" the secretariat of which is held by DIN. The responsible German body involved in its preparation is Subcommittee NA 119-01-03-03 "Mikrobiologie" ("Microbiology") of Working Committee NA 119-01-03 AA "Wasseruntersuchung (CEN/TC 230, ISO/TC 147)" ("Water examination (CEN/TC 230, ISO/TC 147)") at DIN Standards Committee Water Practice (NAW).