Reed, P., Schachtman, T. R., & Hall, G. (1987). Attenuation and enhancement of instrumental responding by signals for reinforcement on a variable interval schedule. Journal of Experimental Psychology: Animal Behavior Processes, 13(3), pp. 271-279. doi:10.1037/0097-7403.13.3.271

Reed, Phil, Todd R. Schachtman, and Geoffrey Hall. "Attenuation and Enhancement of Instrumental Responding By Signals for Reinforcement On a Variable Interval Schedule." Journal of Experimental Psychology: Animal Behavior Processes 13, no. 3 (1987): 271-279.

Reed, Phil, Todd R. Schachtman, and Geoffrey Hall. "Attenuation and Enhancement of Instrumental Responding By Signals for Reinforcement On a Variable Interval Schedule." Journal of Experimental Psychology: Animal Behavior Processes 13.3 (1987): 271-279.