In order to measure the strength of conditioning, a procedure called conditioned suppression is most commonly used. As one of the reflexes elicited by painful stimuli is the suppression of ongoing behaviour, it is possible to measure the strength of association between a neutral CS (e.g. a tone) and a painful US (e.g. an electric shock) by measuring how much an animal's behaviour is reduced in the presence of the CS compared to its absence. If an animal is trained to perform some repeated measurable behaviour, such as pressing a bar in order to obtain food rewards, then the strength of a conditioned emotional response to a separately learned tone-shock association can be determined by measuring the reduction in the animals rate of bar-pressing when the tone is sounded. The measure of the extent to which the CS suppresses responding is called the suppression ratio and is normally defined as being the rate of responding in the presence of the CS divided by the sum of the response rate in the presence of the CS and in the absence of the CS. If A is the response rate during CS and B is the response rate in the absence of the CS (usually measured immediately prior to CS presentation) then the suppression ratio is A/(A+B). With this formula a CS which completely suppresses responding will score 0.0, one that has no particular effect will score 0.5, a stimulus which elevates responding for some reason will score between 0.5 and 1.0. It is important to be clear that although the animal has been trained to bar-press for food before the conditioning experiment begins, and although the effect we measure is on the rate of bar-pressing, conditioned suppression is measuring the strength of the classically conditioned CER - usually a tone-shock association, not a change in the things the animal has learned about bar-pressing for food.