1. Barba, L. S. (2012). Operant variability: A conceptual analysis. The Behavior Analyst, 35, 000–000.
2. Blough, D. S. (1966). The reinforcement of least-frequent interresponse times. Journal of the Experimental Analysis of Behavior, 9, 581–591.
3. Brownfield, E. D., & Keehn, J. D. (1966). Operant eyelid conditioning in trisomy-18. Journal of Abnormal Psychology, 71, 413–415.
4. Cartwright, N. (1981). The reality of causes in a world of instrumental laws. In P. D. Asquith & R. N. Giere (Eds.), PSA 1980 (Vol. 2, pp. 38–48). East Lansing, MI: Philosophy of Science Association.
5. Catania, A. C. (1973). The concept of the operant in the analysis of behavior. Behaviorism, 1, 103–116.