ABX paradigm is a term taken from psychophysics. This paradigm is used when determining difference thresholds between items. In this protocol two stimuli (A and B) are presented to subjects along with a third (X). Subjects are asked to say which stimuli, A or B, matches X more closely. As an example, a subject is shown a picture of a shark (A) and cat (B). They are then shown a picture of a whale (X) and asked which from choice A or B it most closely resembles. As an animal that swims and lives in water, X would more closely resemble the shark.