Skinner's operant conditioning is based through rewards and punishments for behavior. Either positive reinforcement or negative reinforcement may be used as a part of operant conditioning. In both cases, the goal of reinforcement is to strengthen a behavior so that it will likely occur again. A schedule of reinforcement defines which instances of behavior would be reinforced. As the book states, "a schedule indicates what exactly has to be done for the reinforcer to be delivered. A continuous reinforcement schedule is one in which each specified response is reinforced, while a partial reinforcement schedule is one in which only some responses are reinforced. The occurrence of a reinforcement is critical to the learning process and the likelihood of increasing a response.