This International Standard specifies an evaluation procedure for comparing two methods intended for the 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. It does not provide data that would allow an assessment of the precision of the methods being compared. Precision of methods should be assessed as part of their validation. This International Standard does not provide methods for the verification of method performance in a single laboratory.