Methodological considerations in the study of delay discounting in intertemporal choice: A comparison of tasks and modes.

An experimental comparison of two commonly used delay-discounting procedures (binary choice and fill in the blank) and modes of administration (paper and pencil and computer based) was conducted. Statistically significant main effects were found for task type--steeper discounting was observed in the binary-choice task--but not for mode of administration. As simple evidence of validity, hyperbolic curves consistently provided a better fit to the data than did exponential curves for both tasks. Further, magnitude effects were also observed across conditions. Correlational results varied largely as a function of the discounting index (either k or area under the curve) under consideration. Across the two tasks, discounting indices showed modest levels of reliability [r(AUC) = .33; r(k) = .75]. The findings pointed to refinements for both the methodology and criteria that are used to study delay discounting and raised questions about the commonly assumed relationship between discounting and the construct of impulsivity.