Fixed and variable ratios and delays: further tests of an equivalence rule.

A discrete-trial procedure was used to measure pigeons' choices between fixed and variable ratio schedules and between fixed and variable delays before reinforcement. A peck at a green key produced a reinforcement schedule that was constant within a condition but varied across conditions. A peck at a red key produced a ratio schedule (or, in other conditions, a simple delay) whose size was increased or decreased many times a session, depending on the subject's previous choices. The purpose of these adjustments was to estimate an indifference point--a ratio size (or delay duration) at which the subject chose each key about equally often. The results were used to test a simple "equivalence rule" for choices between fixed and variable schedules (Mazur, 1984). This rule, which was applied without using free parameters, predicted the major trends in the obtained indifference points from both ratio and delay conditions. However, some small but consistent deviations from the predictions were apparent. Better predictions were generated with a more complex equation, which included parameters reflecting the subjects' sensitivities to delay of reinforcement and to events of different probabilities. It was concluded that a successful equivalence rule must include parameters that can be adjusted to describe the effects of delay and probability in a given experimental setting. Once these parameters are estimated, however, choices involving both fixed and variable delays and fixed and variable ratios can be accurately predicted with the same equation.