Comparing demand functions when different price manipulations are used: does unit price help?

Six hens pecked a key (Experiment 1) or pushed a door (Experiment 2) to obtain food reinforcement. In both experiments and as an analogue of price changes, the response requirements were varied in two ways: by increasing the number of responses required and by increasing the required force of each response. The two price manipulations (response number and response force) had different effects on behavior and produced different-shaped demand functions when the rates of consumption were plotted logarithmically against the price analogues. Irrespective of response topography, when the number of required responses was varied, the data paths appeared linear, with slopes close to -1.0. When the required force of each keypeck and doorpush was varied, the data paths were clearly curved, with increasingly steep downward slopes as the force increased. Using the concept of unit price did not fully remove the different effects of the two price manipulations. Those differences are best attributed to the differing times needed in order to complete each response unit under those price manipulations.