This International Standard describes the "A" -"not A" test for use in sensory analysis : a) as a difference test, particularly for evaluating samples having variations in appearance (making it difficult to obtain strictly identical repeat samples) or in after-taste (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 sensitivity of an assessor to a particular stimulus. Examples of its application are given in annex A.