The principle of the test consists in presentation to an assessor of a series of samples, some of which are composed of sample "A" while others are different from sample "A"; for each sample, the assessor has to determine whether or not it is identical to "A". This test requires the assessor to have evaluated a known sample "A" prior to the exposure to test samples. Examples of application are given in an annex A. An annex C shows examples of answer forms.