Revisiting the use of two choices in the matching-to-sample procedure.

The use of two choices in the matching-to-sample (MTS) procedure has been discouraged in the literature because it may lead to "reject control," resulting in failures to establish equivalence classes. In the present study, reject control was prevented during training with the two-choice MTS procedure by presenting the correct comparison with one of five possible incorrect comparisons across trials. This procedure was compared to a six-choice MTS procedure, in which these same six comparison stimuli were presented simultaneously across trials. In Experiment 1, conditional discrimination training and emergent relations testing maintained the same number of comparison choices, two or six. Experiment 2 assessed whether training with two or six choices would result in successful tests under a different configuration from the one with which training occurred (i.e., six or two choices, respectively). In Experiment 3, the conditions were the same as in Experiment 2, but minimal instructions were given to the participants. The results showed the establishment of equivalence classes in all test conditions, thus demonstrating success of the different training conditions. The two-choice MTS procedure appears to be at least as effective as the six-choice procedure for training conditional relations and establishing equivalence classes.