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Relative efficacy of cash versus vouchers in engaging opioid substitution treatment clients in survey-based research
  1. Libby Topp1,
  2. M Mofizul Islam2,
  3. Carolyn Ann Day3
  1. 1Viral Hepatitis Epidemiology and Prevention Program, The Kirby Institute, University of New South Wales, Sydney, New South Wales, Australia
  2. 2School of Public Health and Community Medicine, University of New South Wales, Sydney, New South Wales, Australia
  3. 3Central Clinical School, University of Sydney, Sydney, New South Wales, Australia
  1. Correspondence to Dr Carolyn Ann Day, Central Clinical School, University of Sydney, Discipline of Addiction Medicine (C39), Sydney, NSW 2006, Australia;{at}


Concerns that cash payments to people who inject drugs (PWID) to reimburse research participation will facilitate illicit drug purchases have led some ethical authorities to mandate department store/supermarket vouchers as research reimbursement. To examine the relative efficacy of the two forms of reimbursement in engaging PWID in research, clients of two public opioid substitution therapy clinics were invited to participate in a 20–30 min, anonymous and confidential interview about alcohol consumption on two separate occasions, 4 months apart. Under the crossover design, at Time 1, clients of Clinic 1 were offered $A20 cash as reimbursement, while clients of Clinic 2 were offered an $A20 voucher; at Time 2, the form of reimbursement was reversed. Using clinic records to determine the denominator (number of clients dosed), we found that compared with clients offered a voucher, a significantly higher proportion of clients who were offered cash participated in the survey (58% (139/241) vs 74% (186/252); χ2=14.27; p=0.0002). At first participation, respondents most commonly reported planning to purchase food/drinks/groceries (68%), cigarettes (21%) and transport/fuel (11%) with their payments, with those reimbursed in cash more likely to report planning to fund transport/fuel (19% vs 1%; p<.01) and less likely to report planning to purchase food/drinks/groceries (62% vs 76%; p=0.02). Just three out of 155 cash participants reported planning to purchase illicit drugs with their payment. Results demonstrate that modest cash payments enhanced recruitment of this group, an important consideration given the challenges of delineating the parameters of a population defined by illegal activity, seemingly without promoting excessive additional drug use.

  • Informed Consent
  • Substance Abusers/Users of Controlled Substances
  • Research Ethics

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Financial remuneration is standard in research with people who inject drugs (PWID), along with most other human research.1 Reimbursement compensates participants for time and expenses (eg, travel costs), and symbolises their information's value.2 Debate continues over appropriate modes and magnitudes of payments,3 with some arguing that they are rarely warranted.4 Although few written payment guidelines exist in Australia,5 standard cash reimbursement for 30–90 min face-to-face research interviews with PWID is $A20–30. More invasive methodologies (eg, collection of biological samples; sensitive interviews of longer duration) may attract greater reimbursement.

Ethical concerns around reimbursing PWID for research participation have focussed on the potential for cash payments to precipitate new drug use;6 ,7 and/or act as ‘undue inducement’, defined as the provision of incentives of a magnitude that undermines potential participants’ ability to act in their own best interests.8–10 Australian PWID report a number of motivations for research participation including economic gain, expressions of citizenship, altruism, personal satisfaction and/or drug user activism.10 Studies conducted in the USA indicate that cash reimbursement for illicit drug users’ research participation does not encourage additional drug use.6 ,7 ,9 ,11 Rejecting notions of undue inducement, participants recommend that rather than assuming diminished capacity, research regulators view PWID as autonomous decision makers.9

Nevertheless, cash reimbursement for PWID continues to attract scrutiny.12–14 To address perceived ethical complexities around this practice, some protocols reimburse participants with department store/supermarket vouchers rather than cash.11 This approach, denounced by drug user advocacy groups as discriminatory,15 poses distinct ethical challenges. Restrictions on research benefits and incentives for select participant groups conflicts with distributive justice, a key principle underlying health research ethical guidelines.16 Distributive justice demands the fair distribution of research benefits and burdens and equitable treatment of participants. That authorities should seek to dictate how PWID spend their money builds on injecting-related stigma to imply this group's incapacity for responsible spending, stripping participants of their agency, and devaluing their importance and that of the information they provide.14

Scientific validity is a second fundamental principle underlying ethical research conduct.16 Reimbursement is designed to maximise recruitment (increasing statistical precision) and minimise systematic recruitment bias (enhancing external validity).17 Consequently, reimbursement methods which degrade recruitment and result in inadequate response rates may compromise ethical principles of research merit and integrity. As a ‘partially hidden’ population whose parameters are defined by illegal activity, it is impossible to construct representative sampling frames for PWID,18 such that recruitment of large and diverse samples, more likely to approach representativeness, acquires additional imperative.19 However, the literature suggests that compared with cash, vouchers are of less ‘value’ to some drug-dependent populations, and may result in lower follow-up rates.6 ,7 ,20

Most studies of research incentives have been conducted in the USA in the context of longitudinal clinical trial participation. Limited research documents the impact of different incentives in cross-sectional studies. This study was based on the principles of crossover design to test the hypothesis, that relative to a voucher of equivalent value, modest cash incentives would be more effective in attracting Australian opioid substitution treatment (OST) clients, the great majority of whom are PWID,21 to participate in cross-sectional, survey-based research. A secondary aim was to examine potential differences in the way that participants planned to expend their cash or vouchers.


This study was approved by the ethics review committee (RPAH Zone) of the Sydney Local Health District. Researchers attended two geographically distinct OST clinics located within a single health service area during dosing hours on two 3-day periods in July and November 2011. On both occasions, all clients, not in a residential rehabilitation programme, who dosed, were invited to participate in a 20–30 min, confidential and anonymous interview about alcohol consumption. Thus, the primary study aim was not disclosed to participants. Under the study's crossover design, in July, clients of Clinic 1 were offered $A20 cash reimbursement, while clients of Clinic 2 were offered an $A20 voucher. In November, the form of reimbursement was reversed, with Clinic 1 offering the voucher, and Clinic 2 offering cash. Study advertisements placed in clinic waiting rooms 1 week prior to researchers’ attendance advised only that ‘participants are REIMBURSED for their time’. Prior to dosing, a researcher invited all clients to participate, and explained study procedures, including the form of reimbursement offered. Those who consented were interviewed following dosing on or near the clinic premises. Clients could participate only once during each 3-day period. Participants were assured that clinics would receive only aggregate survey results; and their verbal informed consent to participate was obtained. Individuals not dosed by the clinics and/or whose English language skills were insufficient to allow meaningful participation were excluded.

Data analysis

Descriptive statistics were used to characterise participants at their first participation; that is, those recruited in July plus those first recruited in November, the latter group identified through questions about recent survey participation. Clinic dosing records were used to determine the denominator (number of clients dosed) offered cash and voucher reimbursement. Pearson's χ2 tests were used to compare the proportion of clients in each group who (i) agreed to participate and (ii) reported planning to spend their payments on specific items.


Sample characteristics

The mean age of the (n=264) individual participants was 39.1 years (SD±8.48), 56% were men (table 1), and 28% identified as Aboriginal and/or Torres Strait Islander (Indigenous Australian). Fifty-five per cent completed 4+ years of high school education; 97% were in receipt government benefits; and 73% reported previous imprisonment. Thirty-four per cent reported current prescription of mental health medication, and 59% reported a previous mental health diagnosis.

Table 1

Sample characteristics

Ninety-three per cent of participants reported a history of injecting, with a mean age of onset of 19.3 years (SD±5.30). Eighty per cent were engaged in methadone, 12% buprenorphine/naloxone and 9% buprenorphine maintenance. Thirty-six per cent had been engaged in their current OST episode for >2 years. Seventy-two per cent reported illicit drug use in the preceding 6 months, heroin (51%) was the drug used most often during this period. The Severity of Dependence Scale (SDS)22 classified 53% as dependent on the illicit drug they had injected most often in the preceding 6 months.

Relative efficacy and planned spending of cash versus vouchers

Survey participation rates were significantly higher when OST clients were offered cash compared with offers of vouchers (74% (186/252) vs 58% (139/241); χ2=14.27; p=0.0002) (table 2). Among the 203 clients who participated once (ie, under one form of reimbursement only), 65% (125/191) participated for cash reimbursement compared with 43% (78/180) of those reimbursed with vouchers (χ2=18.29; p=0.001). Seventeen per cent of participants who were offered cash reimbursement reported that they would not have participated in the survey had they been offered a voucher. By contrast, just 2% of voucher-reimbursed participants reported they would not have participated in return for cash (19% (29/155) vs 2% (2/109); χ2=17.58; p<0.001).

Table 2

Site-wise participation in individual occasions in two forms of reimbursement

There were few differences in the items on which participants reported planning to spend their reimbursement (table 3). At first participation, respondents most commonly reported planning to use their reimbursements to purchase food/drinks/groceries (68%), cigarettes (21%) and transport/fuel (11%). Substantial minorities also reported planning to purchase miscellaneous items (ie, bills, cell phone credit, rent; 12%), or were unable to nominate specific items (8%). Participants who were reimbursed in cash were more likely to report planning to fund transport/fuel (19% vs 1%; p<0.01) and less likely to report planning to purchase food/drinks/groceries (62% vs 76%; p=0.02). Just 3/155 cash participants planned to purchase illicit drugs, while 2/155 cash and 1/109 voucher participants planned to purchase alcohol.

Table 3

Comparison of planned spending of cash versus vouchers


Our results, derived from empirical observations of 264 OST clients whose characteristics broadly reflect those of Australia's OST client population,23 demonstrate that relative to a voucher of equivalent value, modest cash incentives were more effective in attracting Australian OST clients, the majority of whom are PWID,21 to participate in cross-sectional, survey-based research. Participant self-reports suggested that while modest cash payments enhanced recruitment to this group they did so without promoting excessive additional drug use. Results are thus consistent with previous research demonstrating that cash reimbursements do not encourage additional drug use compared with no cash or non-cash alternatives;6 ,7 ,11 ,24 ,25 and that vouchers are of inherently less value to some research participants, and may, thus, be less effective in engaging such groups in research.20 Given that reimbursement serves to pragmatically demonstrate the value of the information held by participants,2 and also to maximise recruitment and, thereby, scientific validity,19 our findings clearly indicate that authorities seeking to overcome ethical concerns around cash payments to PWID by mandating voucher reimbursement may only raise discrete ethical complexities via the genuine possibility that scientific validity may be compromised through decreased statistical precision and increased recruitment bias.17

In addition to the imperative to implement methods which maximise recruitment of ill-defined populations, we agree with Halpern that the notion that ethics authorities or researchers are better able to determine potential participants’ best interests is patently paternalistic.17 Moreover, restricting the form of reimbursement provided to certain groups of participants encourages, if not enforces, inequitable access to the benefits of research participation and, thus, conflicts with the principle of distributive justice, that is, the equitable distribution of the risks and benefits of research.5 ,14 PWID are a population characterised by poor educational attainment, low employment rates, housing instability and significant disadvantage, and we argue that it is unethical for authorities to selectively mandate restrictions on this group's options for legitimately earning income. Equivalent forms and magnitudes of reimbursements for all research participants, regardless of the population from which they are sampled, are also consistent with the accepted ethical principles of individual autonomy, that is, respecting and not interfering with the actions of individuals; and are one way of recognising and beginning to equalise the power differentials inherent between researchers and their participants.5

This study has some limitations. Self-reported information collected through face-to-face interview may be subject to social desirability bias,25 although the literature suggests PWID reliably report their drug use in non-punitive and non-judgemental environments,26 such as provided by this study. Nevertheless, participants may have failed to disclose their true intentions regarding the expending of their reimbursement. Due to constant turnover in the populations dosing at these two clinics, with new clients entering treatment, and more stable clients progressing to pharmacy dosing, some clients who were recruited at Time 1 were no longer engaged with the services at Time 2. As a result, the study did not constitute a full crossover trial, and differences in the OST client population across sites may have some effect on participation on the form of reimbursement offered. Nevertheless, participation rates were higher for cash than voucher reimbursement at both clinics (table 2), and particularly at Clinic 2, the difference was significant (p<0.001). Overall, our empirical results, together with ethical considerations, suggest that if it is considered appropriate to reimburse survey participants drawn from broader populations with cash, so too should it be with PWID.


We gratefully acknowledge the clinical teams at the two recruitment sites, particularly Professor Paul Haber, Elaine Doherty, Maggie Tynan and Dr Ken Curry, and the research interviewers, Dr Rachel Deacon, Sarah Hutchinson and Lucia Evangelista of Drug Health Services of Royal Prince Alfred Hospital. We thank Dr Md Bayzidur Rahman, Lecturer of Biostatistics, University of New South Wales for his assistance in statistical analysis.



  • Contributors LT and CAD conceived the study. LT secured funding for the study. LT and MMI coordinated the study. All authors assisted with data collection, MMI entered and analysed the data. LT led the writing, and all authors have read approved the final draft.

  • Funding University of New South Wales Faculty of Medicine Research Grant.

  • Competing interests None.

  • Ethics approval Ethics Review Committee (RPAH Zone) of the Sydney Local Health District.

  • Provenance and peer review Not commissioned; externally peer reviewed.

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