Table 1 Checklist for incentive scheme design

1. What are the current incentives and barriers operating in the system?

• Must complement the existing incentives already operating in the setting, and work to overcome the current barriers

• Consideration of the current state of play in a field is a key step but is frequently overlooked and can be affected by availability of information

• What incentives are in the setting already?

• What are the main barriers to recruitment?

• Understanding current systems of incentives operating in trials

2. Who should incentives be directed towards?

• Choice should depend on where the greatest barriers exist, where accountability for improvement lies, and where the greatest gains may be achieved for the available resource

• Whilst there is more evidence to support patient incentives, all options show some promise

• Where do the greatest barriers to recruitment/retention currently lie? With participants, recruiters, or both?

• If the barriers are with recruiters, do individuals have the ability to overcome these, or is a team effort required?

• Testing organisational and individual incentives for recruiters, and shared incentive schemes

3. What should be incentivised?

• Incentives linked to processes generally found to be more effective than outcome-linked incentives, although this evidence is from settings other than trial recruitment and retention

• There must be evidence of a strong causal relationship between the incentivised process and the desired outcome if process-based incentives are to achieve the overall aim

• What is the desired outcome? Recruitment, retention, or both?

• Would linking incentives directly to this outcome transfer unfair risk onto participants or recruiters?

• What processes may lead to this outcome? Is there evidence of a strong causal relationship between processes and this outcome?

• What other outcomes are important? Will these be neglected if not incentivised?

• Testing the relative benefits of process and outcome incentives and of incentivising a single metric compared to a range of measures

4. What form of incentive should be offered?

• The psychological effects of monetary incentives do not appear to crowd out the direct price effect

• Monetary incentives were found to be more effective than non-monetary incentives for participants

• Who is the incentive directed towards? What are they likely to value most or be motivated by?

• Is it possible to provide monetary incentives?

• What is the overall budget for incentive provision?

• Testing of the relative effectiveness of monetary compared to non-monetary incentives for recruiters

5. How large should the incentive be?

• Larger incentives should be more effective

• Size of incentive needed will be very context dependent, increasing in situations that require more effort from participants or recruiters, or more risk

• Incentive size will determine the overall cost of the scheme

• How are agents currently reimbursed?

• How much effort is required from participants or recruiters?

• How large is the risk associated with trial involvement?

• What is the overall budget for incentive provision?

• Would an incentive of the chosen size raise concerns around coercion?

• Testing the cost-effectiveness of larger incentives, accounting for the overall impact on study timelines and costs

6. How should the inventive be structured?

• Incentive structure is crucial in determining the total cost of the scheme

• Most effective structure will vary by the context, and the evidence in this area is sparse

• Evidence suggests there is no difference in effectiveness between guaranteed and lottery-based incentives for patient incentives

• Repeat arrangements with recruiters may warrant exploration of more complex incentive structures

• Who is the incentive directed towards?

• If directed towards recruiters, is this a one-off situation or are repeat arrangements likely?

• What is the overall budget for incentive provision?

• Is budget certainty required from the outset?

• Do agents face different barriers to recruitment and retention?

• Exploration of the effects of more complex incentive structures

7. When, and how often, should payments be made?

• Immediate incentives are generally found to be more effective than those paid out in the future

• The time between the occurrence of the desired behaviour and the incentive should be minimised

• When can incentives be practically provided in the trial?

• Is it possible to provide multiple incentives over time?

• Testing the benefits of multiple incentives over time

8. What are the potential unintended consequences?

• In addition to their impact on recruitment and retention, the introduction of incentives may also result in unintended consequences

• Incentives should be designed to minimise the opportunities for individuals to engage in undesirable behaviours, and potential unintended consequences should be monitored

• Is incentive provision likely to lead to undue inducement or coercion of participants?

• Can exclusion criteria be easily verified?

• Are recruiters likely to game the system?

• What impacts are incentives likely to have in the long run?

• How can opportunities for individuals to engage in undesirable behaviours be minimised?

• What monitoring could be put in place to ensure quality trial conduct?

• Evaluating the extent to which potential unintended consequences materialise in practice