Barba, L. S. (2012). Operant variability: A conceptual analysis. The Behavior Analyst, 35, 213-227.
Baum, W. M. (1974). On two types of deviation from the matching law: Bias and undermatching. Journal of the Experimental Analysis of Behavior, 22, 231-242.
Baum, W. M. (1982). Choice, changeover, and travel. Journal of the Experimental Analysis of Behavior, 38, 35-49.
Brown, P. L., & Jenkins, H. M. (1968). Auto-shaping of the pigeon's key-peck. Journal of the Experimental Analysis of Behavior, 11, 1-8.