less extreme than the law predicts. Undermatching can happen if subjects too often switch between the two response options, a tendency that may be strengthened by reinforcers that happen to occur just after a subject switches. A changeover delay may be used to reduce the effectiveness of such post-switch reinforcers; typically, this is a 1.5 second interval after a switch when no reinforcer is presented. Overmatching is the opposite of undermatching, and is less common. Here the subjects response proportions are more extreme than reinforcement proportions. Overmatching may occur if there is a penalty for switching. A final deviation is bias, which occurs when subjects spend more time on one alternative than the matching equation predicts. This may happen if a subject prefers a certain environment, area in a laboratory, or method of responding. These failures of the matching law have led to the development of the "generalized matching law", which has parameters that reflect the deviations just described. The power law was first shown to fit operant choice data by Staddon (1968) and was generalized by Baum (1974). It has been found to fit a wide variety of matching data. The power law was shown by Mackay (1963) to be derivable from input and output logarithmic function and psychophysical and other behavioral data fitting this model were described by Staddon (1975). This is more conveniently expressed in logarithmic form The constants b and s are referred to as "bias" and "sensitivity" respectively. "Bias" reflects any tendency the subject may have to prefer one response over the other. "Sensitivity" reflects the degree to which the reinforcement ratio actually impacts the choice ratio. When this equation is plotted, the result is straight line; sensitivity changes the slope and bias changes the intercept of this line. The generalized matching law accounts for high proportions of the variance in most experiments on concurrent variable interval schedules in non-humans. Values of b often depend on details of the experiment set up, but values of s are consistently found to be around 0.8, whereas the value required for strict matching would be 1.0. The concurrent VI VI choice situation involves strong negative feedbacks: the longer the subject refrains from responding to an alternative, the higher his probability of payoff: switching is encouraged. Hinson and Staddon (1983) showed that power-law matching always results no matter what the balance between a transient tendency to "stay" vs. a