Can also be defined independently of their behavioural consequences. The effectiveness of a reinforcer depends on the relative probabilities of the responses it occasions and the responses to be reinforced; these can be altered by limiting the organism's opportunities to engage in one or the other response (response deprivation.). If a less probable response produces a stimulus that occasions a more probable response, then the stimulus will reinforce the less probable response. This definition takes into account the relativity of reinforcement. The reinforcement relation is reversible (e.g., if water deprivation makes drink probable than wheel running, the opportunity to drink will reinforce running, but if limited access to the wheel makes running more probable than drinking, the opportunity to run will reinforce drinking).