ISO 8588:2017 specifies a procedure for determining whether a perceptible sensory difference exists between samples of two products. The method applies whether a difference exists in a single sensory attribute or in several. The "A" ? "not A" test can be used in sensory analysis in the following ways: a) as a difference test, particularly for evaluating samples having variations, for example, in appearance (making it difficult to obtain strictly identical repeat samples) or in aftertaste (making direct comparison difficult); b) as a recognition test, particularly for determining whether an assessor or group of assessors identifies a new stimulus in relation to a known stimulus (for example, recognition of the quality of the sweet taste of a new sweetener); c) as a perception test, to determine the ability of an assessor to discriminate stimuli. The "A" ? "not A" test is not appropriate for assessing if two products are sufficiently similar to be used interchangeably (i.e. for similarity testing) because the "A" ? "not A" test inherently involves replicate evaluations of the same products by all assessors. These replicate evaluations violate the basic assumptions for similarity tests to be statistically valid. Examples of its application are given in Annex B. NOTE – Bi and Ennis point out that the estimate of the discriminal distance, d', between the "A" and "not A" samples is the same regardless of the nature of the replicated evaluations performed in the test but that the estimate of the variance of d' does depend on how the replicate evaluations were performed. As such, no general discussion of a Thurstonian analysis of the "A" ? "not A" method, nor of the power of the test is undertaken in this document. Interested readers are referred to Reference  for a detailed discussion of the topic.