ISO 4120:2004 describes a procedure for determining whether a perceptible sensory difference or similarity exists between samples of two products. The method is a forced-choice procedure. The method applies whether a difference can exist in a single sensory attribute or in several attributes.The method is statistically more efficient than the duo-trio test, but has limited use with products that exhibit strong carryover and/or lingering flavours.The method is applicable even when the nature of the difference is unknown (i.e. it determines neither the size nor the direction of difference between samples, nor is there any indication of the attributes responsible for the difference). The method is applicable only if the products are fairly homogeneous.The method is effective for a) determining that either a perceptible difference results (triangle testing for difference), or a perceptible difference does not result (triangle testing for similarity) when, for example, a change is made in ingredients, processing, packaging, handling or storage; b) or for selecting, training and monitoring assessors.