Turning k on its head: comments on use of an ED50 in delay discounting research

Abstract

Delay discounting (DD) describes how the value of a reinforcer decreases as delay to its delivery increases. Relationships between DD and various aspects of drug abuse have been demonstrated reliably. A potential barrier to wider adoption of DD techniques is that results are often expressed in terms that may be too abstract or unfamiliar to a broader audience, particularly when describing or comparing hyperbolic DD functions or values of k. In an effort to potentially make DD results more accessible, the current report explores use of an ED50 value in characterizing DD functions, similar to that used in pharmacology research for characterizing dose-effect functions. The ED50 proposed with regard to DD is the delay that is effective in discounting the subjective value of the delayed reinforcer by 50%. Additionally, a convenient method for calculating ED50 values for DD is discussed.

Fig. 1

Various representations of DD. Top panel shows the relative frequency distribution of baseline k values on a log scale. The curve represents the best-fit normal distribution function. Note that higher k values represent greater rates of DD. Representative k values marked a, b, and c are graphed according to Equation 1 in the bottom panel. Note that steeper, shallower curves indicate greater DD.