Comparison of the triangle and a self-defined two alternative forced choice test

The triangle test and the 2-AFC test or paired comparison are commonly used discrimination tests. The 2-AFC is statistically more powerful than the triangle test, but can only be used when the quality of the difference is known (e.g. one sample has added sucrose and is expected to be sweeter). By allowing subjects to define their own criteria for use in the 2-AFC, the procedure might be used when the sensory difference between two samples is not known or easily defined, as is possible during a warm-up protocol. The research reported here investigated a modification of the sometimes extensive warm-up procedure but limited the number of samplings of the preview samples. This modified procedure we describe as a self-defined 2-AFC (SD-2-AFC). This method was compared to the triangle and conventional 2-AFC using four food systems, differing in the magnitude and quality of sensory differences. The AFC tests (including the SD-2-AFC) performed better than the triangle on a statistical basis. However, in some cases the SD-2-AFC produced lower d-prime values indicating lower absolute levels of sensory differentiation. Correlates of poorer discrimination with the SD-2-AFC included miss-identification of the critical attribute (as determined by majority response) or reversal of the direction of the difference during the inspection phase. Results were interpreted in terms of Thurstonian models. No advantage of adding a preview to the triangle or 2-AFC was observed. The overall statistical advantage of the SD-2-AFC suggests it should be considered as an alternative to the triangle test.