Right-lever presses by hooded rats in three groups produced reinforcers at variable-intervals that differed over five conditions. The groups differed by virtue of the constant rates of reinforcers obtained by left-lever responding-high, medium, or low. The function relating right-response rates to right-reinforcer rates was adequately described by the hyperbola proposed by Herrnstein. According to Herrnstein's interpretation, the rate-of-change parameter (R o ) for the hyperbola represents the rate of reinforcers obtained from sources other than right-lever reinforcers. Estimates of R o did not bear a direct relation to the rate of reinforcers obtained on the left, contrary to what was expected according to Herrnstein's interpretation. There were many instances in which Ro estimates were smaller than the rate ofleft-lever reinforcers actually obtained. Such instances could not be accounted for by Herrnstein's hyperbola unless the undermatching and bias that were found for the choice between right and left reinforcers were taken into account. We concluded that Herrnstein's proposal that response rate can be predicted in terms of the choice between reinforcers arranged by the experimenter and reinforcers obtained from extraneous sources remains tenable if undermatching and bias are taken into account. defined operant response, B, and some other behavior, Bs, is given by where R is the rate of reinforcers obtained by operant responses and R o is the rate of extraneous reinforcers produced by other behavior.Herrnstein (1970, 1974) A dominant theme in contemporary research on operant behavior is that response strength depends on the context for reinforcement. In Herrnstein's (1970Herrnstein's ( , 1974 influential formulation, the reinforcement context for any given behavior is specified by the outcomes of potential choices available to the individual. Herrnstein suggested that in any situation in which the relation between an operant response and its reinforcing consequences is being studied, there always exists some other behavior that is maintained by reinforcers extraneous to the experimentally defmed source of reinforcers. In other words, the individual always has a choice between engaging in operant behavior defined by the experimental procedure versus engaging in some other behavior, whatever that may be.According to Herrnstein (1970), choice between two alternatives is predicted by the "matching law" (de VilHers, 1977;Herrnstein, 1961;Nevin, 1984): response proportions match reinforcer proportions. Following the matching law, the choice between a single experimenter-(1) (

Four pigeons were trained on concurrent variable-interval schedules programmed on a center response key, with access to those schedules controlled by responses on left or right side keys. Two procedures were used. In one, the pigeon was given limited access, in that each side-key response produced 3-s access to a center-key schedule, and in the other procedure, access was unlimited. Data were analyzed using the generalized matching law. Comparison of sensitivities to reinforcement of interchangeover time for both procedures showed them to be of similar magnitude. Response sensitivities were also similar in magnitude for both procedures. From the limited-access procedure a second time measure that was available, switched-in time, was relatively uncontaminated by time spent emitting behavior other than key pecking. Sensitivities to reinforcement for the switched-in time measure were always smaller than interchangeover-time sensitivities for either procedure, and were approximately equal to response sensitivities for the limited-access procedure. Two other access times (5 and 7.5 s) were studied to validate the choice of 3 s as the main access time. These results indicate that when time spent emitting other behavior is excluded from interchangeover time, time and response sensitivities will be approximately equal.