How Two Clean Meth Out Your System

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Health Psychol. Author manuscript; available in PMC 2014 Sep 1.

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PMCID: PMC3940415

NIHMSID: NIHMS536688

Spend today, clean tomorrow: Predicting methamphetamine forbearance in a randomized controlled trial

Kimberly Ling Murtaugh

^bEye for Behavioral and Addiction Medicine, UCLA Section of Family Medicine, Los Angeles, CA 90095-7087

^cTepper School of Concern, Carnegie Mellon Academy, Pittsburgh, PA 15213

Tamar Krishnamurti

^cTepper School of Business organization, Carnegie Mellon Academy, Pittsburgh, PA 15213

^dSection of Engineering and Public Policy, Carnegie Mellon University, Pittsburgh, PA 15213

Alexander L. Davis

^dDepartment of Engineering and Public Policy, Carnegie Mellon University, Pittsburgh, PA 15213

Cathy J. Reback

^eFriends Research Found, Inc, Los Angeles, CA 90038

^fIntegrated Substance Abuse Programs, Semel Institute for Neuroscience and Homo Behavior, UCLA, Los Angeles, CA, 90024

Steven Shoptaw

^bCenter for Behavioral and Habit Medicine, UCLA Department of Family unit Medicine, Los Angeles, CA 90095-7087

Abstract

Objective

This secondary analysis of information 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

82 methamphetamine-dependent gay/bisexual men randomly assigned to conditions delivering CM received monetary vouchers in commutation 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 appurtenances. A time-lagged counting process Cox Proportional Hazards model for recurrent effect survival assay examined aspects of the frequency and blazon of these CM purchases.

Results

Afterward controlling for severity of baseline methamphetamine apply and accumulated CM wealth, every bit measured by cumulative successful earning days, participants who redeemed CM earnings at any visit ("spenders") were significantly more probable to produce stimulant-negative urine samples in the subsequent visit, compared to those who did not redeem ("savers") 1.011* [1.005, 1.017], Z=3.43, p<0.001.

Conclusions

Findings support the economic concept of substitutability of CM purchases and explain trial outcomes equally 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.

Keywords: Contingency Management, Behavioral Economic science, Addiction

Introduction

Although the number of Americans estimated to use methamphetamine in the past thirty days has declined in contempo years, methamphetamine dependence continues to be a significant problem in the Western United States and is the leading substance reported among publicly funded drug treatment episodes in California (California Department of Alcohol and Drug Programs, 2012). Gay and bisexual men utilize methamphetamine at high rates and this utilize has been associated with elevated take chances for HIV infection (Shoptaw & Reback, 2006; Drumright, Gorbach, Little, & Strathdee, 2009; Plankey et al., 2007). Methamphetamine dependence is associated with multiple morbidities including hepatitis, cardiac effects, family disruptions, neurocognitive deficits, and prominent psychiatric consequences (Cruickshank & Dyer, 2009; Gonzales, Mooney, & Rawson, 2010; Scott et al., 2007). Efforts to optimize efficacy of handling for the disorder have great public wellness significance.

To date, just behavioral interventions take shown efficacy for treating methamphetamine dependence, with contingency direction (CM) showing superiority compared to cognitive behavioral therapy during treatment (Shoptaw et al., 2005; Rawson et al., 2006). Contingency direction reflects principles of operant conditioning, reinforcing abstinence from stimulants through delivering increasingly valuable incentives for consecutive urine samples documenting methamphetamine abstinence (see Higgins et al., 1993).

Contingency Management Approaches

Several previous studies have evaluated unlike manipulations of CM payment schedules to increase effectiveness and address toll-effectiveness concerns including changing the magnitude of reinforcement (Carroll et al., 2002; Petry et al., 2004; Silverman et al., 1999; Sindelar, Olmstead, & Peirce, 2007), the rate of reinforcement (Roll, Higgins, & Badger, 1996), and the relative risk of reinforcement (Petry & Martin, 2002; Petry, Martin, Cooney, & Kranzler, 2000). Overall these studies demonstrate robust furnishings of CM to bear on positive wellness behavior alter nether many different situations and reinforcement varietals.

While the success of these approaches is widely recognized, the underlying mechanism is less articulate; specifically why CM is more effective during handling than other forms of behavioral therapy. Information technology is tempting to utilize CM in a largely atheoretical way due to its robust event; withal, to make accurate predictions about behavioral change under a variety of CM reinforcement approaches, it is necessary to understand the theoretical underpinnings of the model.

Applying CM to chronic behavior disorders like methamphetamine dependence is theoretically complex. Creating an association between positive reinforcement and biological markers of drug abstinence instills an active decision-making process on the part of the individual, requiring a balance of executive reasoning and strategies to inhibit want for drug reinforcement. This report draws on behavioral economic and psychological theory to elucidate mechanisms, which may help propose ways to optimize outcomes when using incentive-based behavior therapy. Further, nosotros studied instances of voucher redemption rather than the typical measure of voucher earning to wait more closely at the moment of receiving reward for behavior. Drawing on behavioral economic theory, we examine two contrasting potential mechanisms, substitutability and delay discounting, using patterns of voucher redemption also equally purchase types, in order to explain outcomes of a large trial of CM (Shoptaw et al., 2005) in a sample of methamphetamine-dependent gay and bisexual men in Los Angeles County.

Substitutability

In economic theory, equally in psychology, reinforcers are fixed stimulus events (due east.g., drug use) that tin can strengthen responses which elicit the desired reinforcement and that over time can pb to dependence (Allison & Timberlake, 1974; Premack, 1965). Some goods act as substitutes for one another and although those goods are not identical (e.g., Coke vs. Pepsi), they can be considered qualitatively the same (Rachlin, Battalio, Kagel, & Green, 1981; Rachlin, Greenish, Kagel, & Battalio, 1976). Individuals should be willing to exchange one adept for another if its influence on the reinforcing human relationship is relatively similar (Dark-green & Freed, 1993). Therefore, conceptually a substitute for a drug could be almost anything that an individual would be willing to accept in place of that drug. There may be any number of possible drug substitutes that mimic the desired furnishings of the drug, including other drugs. An effective substitute that tin be implemented in drug dependence treatment is one that does not increase dependency and promotes a healthy, drug free lifestyle. In this enquiry, we propose that vouchers, which correspond to a monetary value, may act as a substitute for methamphetamine by mimicking the hedonic, affective response previously attributed to drug utilise. While a CM voucher is not a perfect substitute for a drug, we advise that the act of spending CM vouchers may provide a reinforcing experience that helps individuals arm-twist a sustained period of forbearance.

Another mode to conceptualize substitutability is in terms of cantankerous-price elasticity (Petry, 2001). Good A is oftentimes chosen (substitutes for) Good B when the toll of Good B increases and the cost of Skillful A does non [e.grand., drinking an alternating soda (Pepsi) when the cost of the preferred soda (Coke) increases]. Costs for methamphetamine tin can accrue in two ways. Get-go, the reinforcing effects of the same corporeality of the drug decrease over fourth dimension due to tolerance, thereby increasing the fiscal price of the drug. Second, experience costs increase with continued utilize in terms of eroded personal, concrete, psychological, and back up factors. When applied to methamphetamine dependence, behavioral economics predicts that provision of a skillful that is experienced as qualitatively similar and comes at a lower cost as the drug should be successful in helping users to switch from their drug to the substitute (CM vouchers). Thus, the efficacy of CM could be explained by the capacity of the vouchers (alone or considering of the appurtenances that vouchers purchase) to compete successfully and qualitatively (i.due east., to substitute) for the reinforcement from methamphetamine apply. From this vantage, it is possible and perhaps probable that individuals who experience a substitution event (i.e., a voucher redemption) will experience an association between the reinforcement of the vouchers and abstinence, thereby predicting better treatment outcomes in the following visit. ⁱ

Based on a substitutability theory of behavior, nosotros would predict that:

H1: If voucher redemptions are interim as a substitute for methamphetamine use, participants who redeem their CM earnings at a visit (spenders) will be more likely to accept a negative urine effect at their adjacent visit compared to participants who do non redeem (savers), even when controlling for baseline methamphetamine dependence and cumulative voucher wealth, as measured by cumulative successful earning days.

The success of a CM program can be affected by the reinforcing relationship between the nature of the good acquired through voucher redemption and drug abstinence. The closer the good is to being a substitute for the drug, the more likely the participant is to develop an operant-conditioned reinforcing relationship between the expert and abstinence. Drugs are a hedonic but brusque-lived feel. While the strength of their hedonic upshot diminishes with chronic use (Ahmed, Kenny, Koob, & Markou, 2002; Koob & Le Moal, 2008), each drug intake event is registered in neurological 'pleasure centers' (Berridge & Kringelbach, 2008; Kringelbach & Berridge, 2009). A perfectly equitable hedonic substitute for a drug may be another drug that stimulates the reward pathways in the brain. Experiencing CM purchases categorized every bit hedonic goods may evoke more pleasure and therefore serve equally a better substitute than utilitarian goods. Every bit such, with respect to the hedonic nature of the appurtenances purchased with CM payments, we would predict that:

H1b: If the appurtenances purchased via voucher redemption are acting as a substitute for methamphetamine, participants who redeem their CM earnings for more hedonic purchases will be more than probable to produce a negative urine sample at their side by side visit compared to participants who redeem their CM earnings for more utilitarian purchases, when controlling for baseline methamphetamine dependence and cumulative voucher wealth, every bit measured by cumulative successful earning days.

Additionally, individuals who abuse drugs tend to disbelieve drugs more steeply than money (for review see Bickel & Marsch, 2001) and moreover, they discount other consumables, such equally nutrient to a similar level as drugs when compared to coin (Odum & Baumann, 2007). Individuals who abuse drugs and have steeper discounting rates for other consumable goods may consider a good that can exist consumed quickly (i.due east., more than similar in consumption charge per unit to methamphetamine) a better substitute compared to one with a long lifespan (i.e., a computer). Therefore, consumable goods rather than durable goods may serve as qualitatively better substitutes for drugs. Individuals tin show domain-specific levels of delay discounting, having like disbelieve rates for goods they personally find tempting, compared to others (Tsukayama & Duckworth, 2010). Individuals in this study were given the freedom to choose their purchases, provided the buy promoted a healthy/pro-social lifestyle (e.thou., rent, music, dry cleaning), according to their desires or needs. With respect to the consumable nature of the goods purchased with CM vouchers then, we would predict that:

H1c: If the appurtenances purchased via voucher redemption are acting as a substitute for methamphetamine, participants who redeem their CM earnings for more consumable purchases will be more likely to requite a negative urine sample at their next visit compared to participants who redeem their CM earnings for more than durable purchases, when controlling for baseline methamphetamine dependence and cumulative voucher wealth, as measured past cumulative successful earning days.

Filibuster discounting

Individuals with drug dependence tend to value immediate rewards more than distant future rewards, compared to those without drug dependence (review past Bickel & Marsch, 2001). This greater caste of delay discounting explains the difficulty of establishing sustained, long-term abstinence. Drug-dependent individuals must make repeated choices betwixt the firsthand reinforcing furnishings of methamphetamine and the distant health, psychological, and social benefits of non using methamphetamine. Those who tin can delay reward (i.due east., successfully resolve the delay disbelieve regarding forbearance) may be more successful in response to a CM intervention. Specifically, individuals who successfully resolve the delay discount, accrue voucher earnings, and make fewer voucher redemptions when money is available (savers), may experience generalized effects that can issue in successful drug forbearance during treatment. Based on a differential delay discounting theory of beliefs, nosotros would predict that, in contrast to H1:

H2: Individuals who delay purchasing over a greater number of consecutive visits (savers), volition be more than probable to produce negative urine results on whatsoever given subsequent visit than those who brand purchases, even when decision-making for baseline methamphetamine dependence and cumulative voucher wealth, every bit measured by cumulative successful earning days.

Method

Participants

Written report participants were 162 methamphetamine-dependent gay and bisexual men who provided voluntary informed consent (reviewed by Friends Research Establish Westward Coast IRB) and were appropriate for outpatient treatment. Participants in this assay included those randomly assigned to atmospheric condition that provided contingency management and who were able to earn whatsoever incentives (north=82; see Figure 1).

Participant menstruation diagram of RCT stages

Procedures

Consented participants entered a 2-week screening period in which they were required to attend clinic and provide a urine sample a minimum of four out of six available times to exist eligible for study participation. Eligible participants were randomized to one of four behavioral therapy conditions: CM only (north=42), cerebral behavioral therapy only (n=40), CM plus cognitive behavioral therapy (n=40) and a gay-specific cerebral behavioral therapy (n=xl) and attended clinic three days per week for 16 weeks to provide urine samples, receive treatment, and complete study measures. Outcomes from the study are described elsewhere (Shoptaw et al., 2005).

Interventions

Contingency Direction

Participants received voucher-based reinforcement therapy with rapid reset after relapse (Higgins et al., 1993). Vouchers were provided for urine samples testing negative for metabolites of methamphetamine starting at $two.50 for the first sample and increasing by $1.25 for successive metabolite-negative urine samples (i.e., $3.75 for the second negative sample; $5.00 for the third). Every tertiary consecutive negative urine sample earned a $10.00 bonus voucher. The maximum that could be earned during the report for providing 100% negative urine samples was Usa $1,277.l. Vouchers earned could exist redeemed for goods or services that promoted a healthy/pro-social lifestyle, such as groceries, gas, utility bills, travel, movies, or music. Participants could redeem at any study visit if they had a positive balance in their account, regardless of their urine upshot at that visit. They could: (i) choose from a set of voucher certificates at the clinic that could be redeemed at specific stores and amusement venues (eastward.g., grocery stores, gas stations, movie theaters), (2) choose any payee by providing a beak to the clinic staff who would write a check, or (iii) be accompanied by a clinic staff member to a store to straight purchase the desired good. When requests were made for staff to back-trail participants to make purchases, requests were typically fulfilled within a 24-hr menses. Only vouchers were provided. No cash was provided.

Cerebral behavioral therapy

Roughly half of the participants (north=37) in these analyses who received contingency management too received Matrix Model cognitive behavioral therapy (Rawson, et al., 1995). This method uses education in a group format to provide instruction and skills related to recovery from methamphetamine dependence.

Measures

Spending Measures

The frequency and magnitude of purchases made with the contingency direction payments were collected during the study. Categorization of participants into spenders or savers was based on a median separate on frequency of purchasing behavior. For specific voucher certificate purchases, the retailer and amount were recorded. When participants received payments of bills or other invoices, copies of the bills and invoices were kept as well as copies of the checks that were written and a description of the purchase was recorded. Using these records, purchases were categorized into the following non-exclusive groups: hedonic/utilitarian and consumable/durable. Appurtenances were categorized equally hedonic if their employ was deemed pleasurable or entertaining (e.k., flick tickets, music) and commonsensical if the expert was deemed practical or necessary (e.yard., gas, prescriptions). Appurtenances were categorized as consumable if they were used completely or became useless in a brusque period of fourth dimension (due east.k., groceries) and durable if their period of usefulness was longer term (e.g., bicycle). An exhaustive listing of goods purchased can be found in Appendix A. Purchase categorizations were determined independently by the outset ii authors and reliability analyses demonstrated high inter-rater reliability (.90 and .96 for the respective categorizations).

Appendix A

Exhaustive listing of appurtenances purchased by categorizations

	Hedonic	Utiliarian
Consumable	Awarding fee for buddhist pilgrimage Magazine/daily newspaper subscription (e.yard., Men's Health Magazine, Body Edifice Magazine, Entertainment Weekly) Movie passes Pic rentals Restaurant nutrient purchases Theater performance tickets Vacation travel (due east.one thousand. airline tickets, rental car)	Public transit tokens Fast food souvenir certificates Gas Groceries Chemist's/drug store purchases Prescriptions
Durable	Art portfolio and fine art supplies/materials Charitable toy donation Cablevision television and net service Estimator purchase Fitness order membership Loftier finish bicycle supplies and accessories High end photography supplies Limousine service Music CD(s) Printer purchase	Automobile expenses (e.g., repair, tires, insurance, machine payment, registration, AAA membership) Clothing Higher tuition Credit carte du jour payments Dry Cleaning Wellness Insurance Abode Goods (e.g., Hold Everything, Pottery Befouled) Medical services (e.1000. dental visits, counseling sessions, residential treatment facility payments) Office supplies Parking violations Pool Cleaning service Public transit passes Rent/HOA payment Storage unit rental Revenue enhancement preparation services Phone services (e.g. cell phone, pager) Used television set circulate equipment Utility services (gas, h2o, power)

Drug Utilize Measures

Baseline methamphetamine dependence was measured by self-written report of days of methamphetamine use within the past xxx days at baseline. Current use was assessed with urine drug screening tests. Observed urine samples were collected at each dispensary visit and analyzed immediately on-site for metabolites of methamphetamine and cocaine to prevent "stimulant switching." If participants provided samples that were positive for either stimulant they did non receive a voucher credit. This was also true for participants who failed to attend the clinic visit or refused to provide a urine sample.

Statistical analysis

The Counting Process version of the Cox Proportional Hazards model for recurrent event survival analysis was used. This method improves upon a cross-sectional arroyo (e.g. Poisson regression) in which the directionality between payment and urine results is unclear (i.e. payment could cause negative urine results or vice versa). Using the Cox Proportional Hazards model, the probability of a negative urine sample at each point is predicted by the covariates from a previous signal and therefore each electric current negative urine sample is solely predicted past past covariates, such every bit cumulative spending to date. This method rules out the take chances that negative urine samples themselves are causing whatsoever model covariate effects found ² .

Further, study visits occurred Mondays, Wednesdays, and Fridays, increasing the likelihood that behaviors on one visit (i.east., voucher redemption) affect the behavior (i.e., acting methamphetamine utilize) recorded at the adjacent visit. The time horizon for the urine analysis to detect metabolites of methamphetamine or cocaine is within the time elapsed between visits. The statistical model calculates the relative risk ratio that a participant will produce a negative urine at any given time point, given a vector of time-independent and fourth dimension-dependent predictor variables that describe the participant'southward prior spending or saving behavior. All models command for baseline methamphetamine dependence, cumulative expenditures, and cumulative successful earning days ³ to account for a participant's acquired wealth in the study at any point.

Assay of covariates

As expected, baseline methamphetamine dependence severity, as measured past number of days of methamphetamine use in the 30 days prior to baseline, was negatively associated with response to treatment every bit measured by negative urine results. Each k days of additional methamphetamine use in the xxx days earlier baseline resulted in 0.965^k*[0.942, 0.989] times lower adventure of having a negative urine sample than a participant without those k days of apply at whatever bespeak during treatment, Z=2.84, p<0.01. For instance, compared to a participant with zero apply at baseline, a participant with one day of use at baseline would exist iii.five% less probable to accept a negative urine result, and a person with ten days of use at baseline would be thirty.0% less likely to have a negative urine result at any indicate during handling.

We examine the effects of both participants' acquired wealth, as measured by cumulative successful earning days, and total expenditures on negative urine results at whatsoever given point. First, cumulative successful earning days were added to the model as a time-varying factor to account for the corporeality of wealth that a participant had accumulated to date. As expected, for each grand additional clean tests in the previous time periods, a participant had 1.186^grand*[1.155, 1.219] higher risk of a negative urine sample in the subsequent study visit, Z=12.4, p<0.001. For example, a participant with 1 negative urine result to engagement would exist 1.nineteen times more likely to have a negative urine result in the electric current visit, and someone with ten negative urine results to engagement would be 5.51 times more likely to accept a negative urine result in the current visit compared to one with zero negative urine results to date.

2nd, a model including only cumulative prior expenditures as a time-varying factor accounted for the amount of wealth that a participant had spent to date. As expected, for each k additional dollar spent in the previous time periods, a participant had 1.008^k* [ane.007, one.009] higher risk of a negative urine sample in the subsequent visit, Z=6.75, p<0.001. For instance, compared to a participant who had spent zero dollars to date, a participant who had spent x dollars to appointment was i.083 times more likely to take a negative urine results in the current visit, and someone who had spent one hundred dollars to engagement was ii.218 times more likely to have a negative urine at the current visit.

To dissimilarity the effects of acquired wealth, as measured past cumulative successful earning days, and cumulative prior expenditures, nosotros entered both into the model. Nevertheless, cumulative prior expenditures was unrelated to take a chance of negative urine samples after decision-making for baseline use and cumulative successful earning days p>0.05. Both covariates are retained for all subsequent models.

The marginal effects presented in the following models are all valuated at hateful levels of covariates: baseline methamphetamine utilize (9.74 days), cumulative successful earning days (15.93 days), and cumulative prior expenditures (United states of america$134.20).

Results

Sample demographics

This analysis included men with a mean historic period of 36.4 years (S.D.=vii.0), with 14.6 (Due south.D.=2.five) years of education who earned per month on average The states$2450.09 (Southward.D.=U.s.a.$3250.22), and spent US$248.35 (Southward.D.=388.06) on methamphetamine in the 30 days prior to baseline. In all, 59.eight% of the participants were HIV positive, 76.8% identified as Caucasian, 17.ane% Hispanic, 3.vii% Asian/Pacific Islander, and ii.four% African-American.

Overall methamphetamine use outcomes

Equally reported in Shoptaw et al. (2005), participants in the CM weather showed significant reductions in urine-verified methamphetamine use during treatment compared to a standard cognitive behavioral therapy condition. No differences were institute between the two CM conditions in overall negative urine samples (CM 25.95 (Due south.D.=16.37), CM+CBT 32.33 (Due south.D.=16.38) t(80)=−ane.76 p>0.05). Additionally, no differences were institute in spending behaviors betwixt participants receiving CM only or CM + CBT (Mean purchases_CM-simply = 7.24 and Hateful purchases_CM+CBT = five.xl, F(1,67) = 1.17, p = .28). Thus, for the purposes of examining the furnishings of spending and saving behavior on negative urine samples both CM groups are collapsed.

Overall Spending Behaviors

Participants averaged a full of half dozen.iii unique purchases with their earnings (Due south.D. = seven.1) and had earned an average of US$557.61 past the end of the study (S.D. = U.s. $468.46), or 43.seven% of the full possible. The distribution of number of full purchases was bimodal and supported creating two groups of purchasers. A median split divided the participants into spenders (northward=forty), who made an average of 10.0 purchases (S.D.=seven.iv), which was significantly more than than the savers (north=42), who made an average of 0.nine purchases (S.D.=0.7), t(80)=eight.00, p<.001. Figure 2 shows a trend over time for spenders to do better in treatment over time compared to savers. To clarify this trend, we must control for our covariates to a higher place which is accomplished using the lagged counting process version of the Cox Proportional Hazards model for recurrent event survival analysis.

Negative urine results trends over time for "spenders"and "savers"

Hypothesis1: Substitutability

Consistent with hypothesis1, making a purchase at any visit significantly decreased the risk of a positive urine result at the subsequent visit, even afterward controlling for covariates. Making a buy in the previous time catamenia significantly increased the likelihood of providing a negative urine issue in the subsequent visit 1.011* [one.005, 1.017], Z=3.43, p<0.001. For example, a participant with baseline methamphetamine use of nine.7 days, with 15.ix cumulative make clean urines, who has spent $134.20 to date, was significantly (ane.011 times) more probable to produce a negative urine sample on the next visit than if they had non made a purchase.

Hypothesis 1b: Purchase types: Hedonic vs. Commonsensical

The hedonic or utilitarian nature of the purchase fabricated in the previous time period did have an effect on the likelihood of a negative urine test in the subsequent period. Participants who fabricated a hedonic purchase in the previous visit were significantly more likely (0.42%) to provide a negative urine sample than those who made no buy ane.004* [1.000,1.008], Z = 2.06, p <0.04. Similarly, those who made a utilitarian purchase in the previous visit were significantly more likely (0.33%) to provide a negative urine sample than those who made no purchase 1.003*[1.000, 1.006], Z = ii.19, p < 0.03. There were no statistically pregnant differences between the hedonic or commonsensical types of goods purchased, however, on the likelihood of providing a negative urine sample Z=0.35, p=0.72.

Hypothesis 1c: Purchase types: Consumable vs. Durable

Participants who made a consumable purchase in the previous visit were significantly more than likely (0.68%) to provide a negative urine sample than those who made no buy 1.007*[1.002, one.011], Z = 3.02, p = 0.003. Similarly, those who fabricated a durable purchase were significantly more likely (0.sixty%) to provide a negative urine sample than those who fabricated no purchase 1.006* [1.003, ane.009], Z = 3.55, p < 0.001. There were no statistically significant differences betwixt the consumable or durable nature of goods purchased on the likelihood of providing a negative urine sample Z=0.28, p=0.78.

Hypothesis 2: Filibuster Discounting

Contrary to hypothesis two, participants who saved their CM earnings, by choosing not to purchase on days even when money was bachelor, were significantly more than probable to have stimulant positive urine results compared to participants that did not salve. The longer participants delayed purchases when they could have purchased something (with a monetary voucher balance greater than naught), the less likely they were to provide a negative urine result at their adjacent visit 0.937*[0.870, 1.011], Z = 1.68, p = 0.09. In fact, each boosted day of delaying reward was associated with a half dozen.26% decrease in the likelihood of providing a negative urine sample at the subsequent visit.

Discussion

These findings show that it is the act of voucher redemption, rather than the receipt of payment that affects subsequent methamphetamine abstinence. Further, frequent voucher redemption regardless of buy size was related to methamphetamine forbearance. Finally, participants who delayed gratification and demonstrated streaks of saving by non redeeming even when vouchers were bachelor had worse treatment outcomes one time they finally redeemed their vouchers. As such, these methods and findings are novel and contribute new findings to existing CM research that focuses on commitment, likelihood, and magnitude of earnings.

While much work has been conducted showing the effectiveness of CM, examining the furnishings of altering various aspects of the rewards provided in CM (such as manipulating the magnitude of reward), very little work has focused on the role that the participants' psychological response plays in their redemption of these rewards and subsequent success in CM programs. We tested two of these psychological mechanisms that could be responsible for the success of CM vouchers (substitutability and delay discounting). Our piece of work is novel in that it presents a set of analyses founded in behavioral economic theory that extend the findings of the randomized controlled trial by demonstrating that it is actually more important for individuals to spend their CM payments than to save them.

Our argument depends on two assumptions: (1) that for gay and bisexual men with methamphetamine dependence, inbound handling happens when the baseline experience of methamphetamine use no longer exceeds the discounted value of accumulating adept wellness; and (two) earning vouchers during the trial accounts for the extent to which people value the voucher relative to new uses of methamphetamine. If these are true, then the caste to which one works to earn a voucher past abstaining from the drug during the trial should be completely dependent on the discounted value of the voucher and good health relative to the current value of using the drug, both of which are captured in baseline dependence and periods of abstinence over time during the trial. If the machinery of the vouchers in eliciting abstinence in the trial is purely a reward function, the act of spending the voucher should brand no difference above and beyond these ii controls.

The substitutability model we propose holds that spending the voucher non simply redeems the value of the voucher, but the human activity also temporarily competes for the subjective experience of the drug. Our statistical analyses showed that using the voucher in the previous menses reduces the probability of using the drug in the side by side period above and beyond their acquired wealth, as measured by their cumulative successful earning days, too as their discount charge per unit and prior value of the drug (captured past baseline dependence and negative urines over the trial). We posit that these results indicate a reduction in the value of the drug as explained by a model of substitutability.

Participants who spent their incentive earnings were more than probable to be abstinent at their adjacent visit simply a few days after compared to those who had coin simply did not spend their earnings, which strongly implicates the reinforcing ability of the purchase. Although the relative risk ratios appear modest at hateful levels of the covariates, the effect becomes chop-chop relevant to success over the course of the treatment. Measuring a greater than 1% comeback in negative urine results is practically meaningful, given that this assay controls for baseline utilize (i.e., severity of addiction), cumulative success in the task, and cumulative expenditures in the prior weeks. While the effect of each individual spending or saving consequence seems modest, over the 16 weeks of handling participants that demonstrated spending behavior had better treatment outcomes overall compared to those who did not spend. These findings lend support to a substitutability explanation of CM when used as a handling for dependence and provides no back up for reducing the delay discount inherent in stimulant treatment.

In these analyses, participants who saved their wealth thereby delaying their reward and demonstrating less steep levels of delay discounting, were really less likely to take a negative urine sample after they made their greater value purchase. Further, the longer participants remained abstemious without making a purchase, the more likely they were to have a positive urine sample at any given visit. These findings suggest that reducing levels of delay discounting via monetary saving ultimately fails to compete with the acute reinforcing effect of methamphetamine and over time becomes counter-therapeutic to the goal of sustained forbearance. In addition, purchasing any type of good (hedonic, commonsensical. consumable, and durable) had a positive and significant outcome on beingness drug abstemious at the next visit compared to those who did non buy, providing support for the finding that the act of purchasing has an consequence on handling outcomes. However, no one category of goods was found to have significant contribution to handling outcomes compared to other types of goods.

There are several limitations to this work. Categorizing goods as hedonic or utilitarian is quite challenging, as this categorization tin can be extremely subjective and individual-specific (due east.g., purchasing an item of habiliment may be a pleasurable ownership feel for ane individual simply a practical one for another individual). Thus, the authors acknowledge that categorization of goods equally hedonic or consumable types to be a limitation to this work. In future piece of work, it would exist important to ask participants to charge per unit the hedonic or utilitarian nature of their purchase. To truly examine how the nature of the purchase, independent from the purchaser, might impact adherence to handling, one would ideally use a randomized controlled trial to assign participants to make a hedonic or consumable purchase of their choosing. However, the choice freedom in the study design does point that the appurtenances purchased were particularly appealing to the purchaser, increasing their likelihood of serving every bit a substitute for methamphetamine. Moreover, it is possible that at that place is some agency or cocky-efficacy component of choice liberty in purchase selection that reinforces the effect.

A 2d limitation to this work was the inability to measure the participant's complete economic portfolio. It is possible that in some instances participants practical their report earnings towards purchases that did non act as substitutes, freeing up resources from other income sources to make purchases of drug substitutes that could non exist measured in this study. Yet, as an of import first stride at agreement the underlying mechanism explaining the difference between more and less successful participants in this study, farther research on purchase types every bit experimental variables could shed more calorie-free on these questions.

Lastly, it is possible that choosing to spend a voucher captures features of the disbelieve rate that are non captured past prior uses of the drug. However, those who have a lower disbelieve charge per unit should be more willing to abstain to earn the voucher but less willing to spend the voucher in one case it is earned, which would imply the opposite pattern of results from what we obtained. Some other possibility is that earning vouchers in the trial does non fully capture the value of the voucher. Thus, participants would work more to earn vouchers and besides spend them because they value them more, and as a issue keep to abstain to earn more vouchers. Although possible, it seems implausible that this cistron would not exist captured by cumulative success in the trial. A more complex problem involves the dynamic interplay between saving vouchers and the irresolute value of the drug. Information technology may be possible for participants to strategically abstain from the drug to reduce their tolerance while saving up vouchers for larger purchases, and then using the drug when one has lower tolerance and compounding that reinforcement with buying large purchases. This is unlikely, yet, as all of these participants met criteria for dependence, which is defined by prior failed attempts to quit or cut down drug use.

Two of the main barriers to broad-calibration implementation for CM are the perceived high cost, despite studies demonstrating its price effectiveness (Sindelar, Olmstead, & Peirce, 2007), and moral objections that public sources of funding should not pay people to behave the way they "should" already. In implementing public health policy, however, science and evidence provide solid directions for addressing public health problems, which contrasts greatly with responding to moral objections, especially when there is a lack of effective treatments. Our findings propose that the act of voucher redemption itself is plenty to drive successful CM interventions, something that has not been systematically observed previously. Bringing behavioral economic theories to join with this new examination of the data shed light on mechanisms that may be used to optimize incentive programs for drug dependence and perchance other problem behaviors. Emphasis on purchasing behaviors during incentive-based study design and implementation could improve the precision and impact of using this method, particularly when translating CM interventions into real globe applications.

Acknowledgments

The authors gratefully acknowledge support of NIDA grants R01DA11031 and T32DA26400. Drs. Shoptaw and Reback acknowledge additional support from the National Institute of Mental Health (P30 MH58107).

Footnotes

^aneReceiving a voucher does increase the individual's overall wealth. If participants were to view vouchers as fully fungible with their other wealth, receiving or spending a voucher should increase the availability of participant's personal money to spend on drugs.

²It is important to note that while The Cox Proportional Hazards model does rule out reverse causality, it does not rule out the possibility of a mutual cause of both the DV and the covariates. This model holds that the probability of having a positive urine sample is a function of the overall risk of a positive urine sample for all participants at baseline level of covariates (the take a chance function), multiplied by a prepare of covariates that can modify this risk. This multiplicative (proportionality) assumption allows ane to determine whether each covariate changes the risk of having a positive urine sample without regard to the unknown hazard function. Parameters for each covariate are estimated by seeing whether a positive urine sample occurs at different levels of covariates for each event accumulation beyond all participants only ignoring possible inside-participant correlations. The Counting Process model assumes that the order of events does not thing and models each period separately by plumbing equipment a Cox PH model to the unabridged dataset (Andersen & Gill, 1982; Therneau & Lumley, 2009, Twisk, et al., 2005). The standard errors of parameters are and then calculated to accommodate for within-participant clustering past using a grouped jackknife guess that repeatedly drops ane participant from the statistical assay and re-calculates the parameters of the model (Thorneau & Grambsh, 2000). The variability of each model parameter across all ane-participant deletions is then the robust guess of the variance of the parameters.)

³Equally an culling to using cumulative successful earning days, analyses were performed using cumulative cash balance. Nonetheless, these models did not produce significantly different results.

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Source: https://www.ncbi.nlm.nih.gov/pmc/articles/PMC3940415/

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