Effects of two sorting formats and four test criteria on equivalence class formation

The likelihood of forming equivalence classes was influenced by the format used in sorting tests and by four different test criteria applied to the same data set.  After 30 participants learned 12 conditional discriminations, MTS tests evaluated the emergence of three 5-member equivalence classes. These tests were followed by sorting tests that were conducted in clustering or stacking formats. After training, 20% of participants formed the classes. Of the 75% who did not, classes emerged for 36% and 15% of participants during stacking and clustering, respectively, with a criterion of consecutive class-indicative sorts in the first two sorting tests, and by 45% and 15% of participants during stacking and clustering, respectively, with a criterion of two successive class-indicative sorts in any of the four sorting tests. Overall, a somewhat higher percentage of participants formed classes during stacking than during clustering, sometimes on a delayed basis. Finally, even higher yields were obtained when criterion was defined as two nonconsecutive class-indicative sorting tests. When classes did not form, clustering rather than stacking tests generated larger proportions of stereotyped, participant-defined, three-member classes and two-term relations but stacking generated more one-stimulus "groupings." Thus, class formation was influenced by sorting format and the criteria used to define class emergence. Also, sorting influenced performances even during failed class formation.