ISO 10399:2004 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 is applicable whether a difference exists in a single sensory attribute or in several attributes.The method is statistically less efficient than the triangle test but is easier to perform by the assessors.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 (duo-trio testing for difference), or a perceptible difference does not result (duo-trio testing for similarity) when, for example, a change is made in ingredients, processing, packaging, handling or storage; or b) for selecting, training and monitoring assessors.Two forms of the method are described: the constant-reference technique, used when one product is familiar to the assessors (e.g. a sample from regular production), and the balanced-reference technique, used when one product is not more familiar than the other.