Small and large fixed ratios with the same unit price

Key pressing of rats was maintained under multiple and discrete-trial choice schedules with reinforcer units of 45 mg food pellets or 3.5 s dips of sucrose solution. Both smaller and larger fixed ratio (FR) schedules were associated with the same unit price in a manner, for example, that each of eight iterations of FR120 was associated with delivery of a single reinforcer unit and one instance of FR960 was associated with eight reinforcer units. FR requirement varied between 20 and 1560 per aggregate reinforcer and unit price varied between 20 and 240 per reinforcer unit. During multiple schedules with food reinforcers, rates and patterns of responding were comparable over nearly a 50-fold range of FR requirements (20-1380) when unit price was 20; over nearly a six-fold range of FR requirements (120-720) when unit price was 120; and was only marginally maintained when unit price was 240. Demand for food pellets was comparatively inelastic at FRs between 20 and 120, during which subjects did not receive supplemental feeding outside experimental sessions, but was elastic at FRs greater than 240, when subjects sometimes did receive supplemental feeding. In a discrete-trial choice procedure with a constant unit price of 120 for sucrose solution, subjects were indifferent between smaller FRs and alternative FRs as large as 480, but began switching away from larger FRs that were 600 or greater. Because responding had been comparably maintained under both FR120 and FRs as large as 960 in the multiple schedule, results from the choice procedure indicated that choice performance was influenced by variables other than FR requirement and unit price. Because aggregate reinforcers were the same for smaller and larger FRs, the most likely reason for preferring smaller FRs was the nearness in time to some reinforcer.