Unit-price analysis of opioid consumption by monkeys responding under a progressive-ratio schedule of drug injection.

Several reports have indicated that drug consumption in self-administration procedures is a function of the ratio of the instrumental requirement to the dose of drug, a quantity termed unit price. We evaluated three predictions from this unit-price model in a reanalysis of data on opioid self-administration in rhesus monkeys responding under a progressive-ratio schedule (Hoffmeister, 1979). We evaluated whether consumption was inversely related to unit price, and compared the goodness of fit of an equation devised by Hursh, Raslear, Shurtleff, Bauman, and Simmons (1988) to that of a linear model predicting consumption as a function of dose. We also tested whether consumption was constant when the same unit price was comprised of different combinations of dose and instrumental requirement. Consumption declined overall as unit price increased. The equation devised by Hursh et al. and the linear model based on dose fit the data equally well. Drug consumption was not uniform at a given unit price. The analyses suggest limits on the unit-price model as a characterization of drug consumption.