In behavioral psychology, reinforcement is a consequence applied that will strengthen an organism's future behavior whenever that behavior is preceded by a specific antecedent stimulus. This strengthening effect may be measured as a higher frequency of behavior (e.g., pulling a lever more frequently), longer duration (e.g., pulling a lever for longer periods of time), greater magnitude (e.g., pulling a lever with greater force), or shorter latency (e.g., pulling a lever more quickly following the antecedent stimulus). There are two types of reinforcement, known as positive reinforcement and negative reinforcement; positive is whereby a reward is offered on expression of the wanted behaviour and negative is taking away an undesirable element in the persons environment whenever the desired behaviour is achieved. Rewarding stimuli, which are associated with "wanting" and "liking" (desire and pleasure, respectively) and appetitive behavior, function as positive reinforcers; the converse statement is also true: positive reinforcers provide a desirable stimulus. Reinforcement does not require an individual to consciously perceive an effect elicited by the stimulus. Thus, reinforcement occurs only if there is an observable strengthening in behavior. However, there is also negative reinforcement, which is characterized by taking away an undesirable stimulus. Changing someone's job might serve as a negative reinforcer to someone who suffers from back problems, i.e. Changing from a labourers job to an office position for instance.