Comparison of d′ estimates produced by three versions of a duo-trio test for discriminating tomato juices with varying salt concentrations: The effects of the number and position of the reference stimulus

The effects of the number and position of the reference stimulus given in the duo-trio test were investigated using tomato juice with various salt concentrations in a roving discrimination design. Estimates of d′ indicated a superior performance of the duo-trio when the reference was tasted twice, first and last as a reminder (DTFR), when compared with the results of the traditional duo-trio (DTF) and the duo-trio with the reference tasted in the middle (DTM). In the DTFR and DTM, the sequence of the last two stimuli in the test had a significant effect on performance, probably due to its aid in identifying the sensory dimension of difference. Comparison of the performance of the DTF and DTM tests for sequences having the same ordering (WWS or SSW) revealed that d′ estimates tended to be higher for the DTF, despite the suggestion that the memory requirement is reduced in the DTM. These results were explained successfully by the Sequential Perception Analysis (SPA) model.