For each task two outcome variables were of primary interest: discount rate and time to completion. Discount rates for the adjusting amount task were obtained by obtaining the best-fit parameters of Equation 1 drawn through the indifference points in Microsoft Excel 2010 (Redmond, WA, USA). A modification of criteria proposed by Johnson and Bickel (2008) (link) to evaluate logical consistency of discounting were used to exclude data: participants’ 1-day indifference point needed to be at least $100 greater than their 25-year indifference point and no more than one indifference point could be more than $200 greater than the indifference point preceding it. Five participants were excluded for failing to meet these criteria. All five of these participants’ data were also not well fit by Equation 1 , with r2 values all < 0. One additional participant was excluded despite meeting the Johnson and Bickel (2008) (link) for also yielding an r2 < 0 since a k value obtained in such cases is not representative of a discount rate present in the underlying data, for 105 total participants for the adjusting amount task. Discount rates for the 5- trial adjusting delay task were calculated by taking the inverse of the obtained ED50 value, which is equivalent to the value of k in Equation 1 when fitted to this singular point. Discount rates were log-transformed for all data analyses, although the absolute discount rates are presented in this paper for clarity. Time to task completion was calculated from the moment the instruction screen was dismissed for each task to the time that the choice was made by the participant in the final choice trial.
Statistics were computed in IBM SPSS Statistics 21 (Armonk, NY, USA). Discount rates and time to completion for the different tasks were analyzed with generalized linear models using exchangeable correlation structures and generalized estimating equations to account for intrasubject correlation in repeated-measures data (Liang & Zeger, 1986 ). Participant gender, age, race, and number of drinks per week were entered into the statistical model along with task type. Cigarette smoking status was not entered into the model due to the low incidence of cigarette smoking in this sample (4%). Significance values for pairwise comparisons between tasks were adjusted with the sequential Bonferroni procedure to keep the family-wise Type 1 error rate to 0.05%. Reported p values reflect this adjustment. The principal component analysis was conducted in with the minimum Eigenvalue set to 1 and Varimax rotation.
Statistics were computed in IBM SPSS Statistics 21 (Armonk, NY, USA). Discount rates and time to completion for the different tasks were analyzed with generalized linear models using exchangeable correlation structures and generalized estimating equations to account for intrasubject correlation in repeated-measures data (Liang & Zeger, 1986 ). Participant gender, age, race, and number of drinks per week were entered into the statistical model along with task type. Cigarette smoking status was not entered into the model due to the low incidence of cigarette smoking in this sample (4%). Significance values for pairwise comparisons between tasks were adjusted with the sequential Bonferroni procedure to keep the family-wise Type 1 error rate to 0.05%. Reported p values reflect this adjustment. The principal component analysis was conducted in with the minimum Eigenvalue set to 1 and Varimax rotation.