Both the triangle test and 2-AFC are commonly used discrimination tests used in research and industry. The 2-AFC is statistically more powerful than the triangle test, but can only be used when the quality and direction of difference is known (ie one sample is sweeter). By allowing subjects to define their own criteria for use in the 2-AFC, it has been suggested, but not proven, that the 2-AFC procedure can be used when the sensory difference between two samples is not known or easily defined. This modified 2-AFC procedure is called the Subject Defined 2-AFC (SD-2-AFC). This study compares the SD-2-AFC to the triangle and conventional 2-AFC (2-AFC) under a number of realistic conditions. Four food systems, different in the magnitude and quality of sensory difference, were examined. Results demonstrated that, in conditions where the 2-AFC could have been used, the SD-2-AFC did not perform as well as either the 2-AFC or triangle tests. In conditions where the 2-AFC could not have been used, the SD-2-AFC did not perform as well as the triangle test. The failure of the SD-2-AFC was due to subjects inverting their criteria (ie, picking'less sweet'instead of'sweet'). Inverted subjects performed lower than expected, whereas noninverted subjects performed better than the 2-AFC or triangle tests. These results were explained in terms of signal detection theory.