Objective: This secondary analysis of data from a randomized controlled trial tested two behavioral economics mechanisms (substitutability and delay discounting) to explain outcomes using contingency management (CM) for methamphetamine dependence. Frequency and purchase type (hedonic/utilitarian and consumable/durable) of CM payments were also examined.
Methods: A total of 82 methamphetamine-dependent gay/bisexual men randomly assigned to conditions delivering CM received monetary vouchers in exchange for stimulant-negative urine samples in a 16-week trial requiring thrice weekly visits (Shoptaw et al., 2005). At any visit participants could redeem vouchers for goods. A time-lagged counting process Cox Proportional Hazards model for recurrent event survival analysis examined aspects of the frequency and type of these CM purchases.
Results: After controlling for severity of baseline methamphetamine use and accumulated CM wealth, as measured by cumulative successful earning days, participants who redeemed CM earnings at any visit ("spenders") were significantly more likely to produce stimulant-negative urine samples in the subsequent visit, compared with those who did not redeem ("savers") 1.011* [1.005, 1.017], Z = 3.43, p < .001.
Conclusions: Findings support the economic concept of substitutability of CM purchases and explain trial outcomes as a function of frequency of CM purchases rather than frequency or accumulated total CM earnings. Promotion of frequent purchases in incentive-based programs should facilitate substitution for the perceived value of methamphetamine and improve abstinence outcomes.
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