Discriminability is generally defined as the ability to distinguish between one stimulus and another. In the Recognition-by-components theory of object perception, Discriminability is a property of geons, which indicates that each geon can be distinguished from other geons from almost all viewpoints. In item analysis, Discriminability is how well an item performs in relation to some criterion. For example, items may be compared according to how well they separate groups who score high and low on the test. The index of discrimination would then be the correlation between performance on an item and performance on the whole test.