1. Secondary reinforcement strength as a function of schedule of primary reinforcement;Armus;J. Comp. Physiol. Psychol.,1961
2. Training that clicks;Briggs;Equus,1999
3. Burgess, K., 1968. The Behavior and Training of a Killer Whale (Orcinus orca) at San Diego Sea World. Int. Zoo Yearbook, 8, 202–205
4. Chance, P., 1994. Learning and Behavior, 3rd. Edition. Brooks Publishing Co., Pacific Grove, CA. p. 217.
5. Food-tokens as incentives for learning in chimpanzees;Cowles;Comp. Psychol. Monographs,1937