Final answer:
A variable schedule of reinforcement is when behavior is reinforced after an average number of times, but on an unpredictable basis. An example of this is gambling on a slot machine.
Step-by-step explanation:
A variable schedule of reinforcement is when behavior is reinforced after an average number of times, but on an unpredictable basis. This means that the reinforcement does not occur after every response, but rather after an unpredictable number of responses. An example of a variable schedule of reinforcement is gambling on a slot machine, where the rewards (money) are delivered on a variable ratio reinforcement schedule. The occasional rewards make the behavior very persistent and difficult to extinguish.