Area under the curve as a novel metric of behavioral economic demand for alcohol

Abstract

Behavioral economic purchase tasks can be readily used to assess demand for a number of addictive substances, including alcohol, tobacco, and illicit drugs. However, several methodological limitations associated with the techniques used to quantify demand may reduce the utility of demand measures. In the present study, we sought to introduce area under the curve (AUC), commonly used to quantify degree of delay discounting, as a novel index of demand. A sample of 207 heavy-drinking college students completed a standard alcohol purchase task and provided information about typical weekly drinking patterns and alcohol-related problems. Level of alcohol demand was quantified using AUC--which reflects the entire amount of consumption across all drink prices--as well as the standard demand indices (e.g., intensity, breakpoint, Omax, Pmax, and elasticity). Results indicated that AUC was significantly correlated with each of the other demand indices (rs = .42-.92), with particularly strong associations with Omax (r = .92). In regression models, AUC and intensity were significant predictors of weekly drinking quantity, and AUC uniquely predicted alcohol-related problems, even after controlling for drinking level. In a parallel set of analyses, Omax also predicted drinking quantity and alcohol problems, although Omax was not a unique predictor of the latter. These results offer initial support for using AUC as an index of alcohol demand. Additional research is necessary to further validate this approach and to examine its utility in quantifying demand for other addictive substances such as tobacco and illicit drugs.

Figure 1. Representative Demand and Expenditure Curves

Panel A depicts demand curves from two hypothetical individuals exhibiting high demand (filled markers) and low demand (unfilled markers). The light gray shaded region reflects AUC for the low demand individual while AUC for the high demand individual is the sum of the dark and light gray shaded regions. Panel B depicts the corresponding expenditure curves. Drink prices are presented along the x-axis, with logarithmic scale used for proportionality. Demand indices and AUC values are also indicated. Note, the logarithmic scale of the x-axis results in a visual distortion of AUC due to stretching of low price intervals.

Figure 2. Demand and Expenditure Curves on the Alcohol Purchase Task

Panels A and B depict demand and expenditure curves, respectively, from the alcohol purchase task assessment. Data points reflect the sample mean value at each price with error bars depicting standard error of the mean.