Cumulative Record. Chapter 10: The Experimental Analysis of Behavior. Quote 13

A variable-ratio schedule programmed by a counter corresponds to the variable interval schedule programmed by a timer. Reinforcement is contingent on a given average number of responses but the numbers are allowed to vary roughly at random. We are all familiar with this schedule because it is the heart of all gambling devices and systems. The confirmed or pathological gambler exemplifies the result: a very high rate of activity is generated by a relatively slight net reinforcement. (p. 138)