Key findings
A total of 22% of those offered no incentive enrolled in the smoking cessation study. In contrast, 36% offered $200 agreed, as did 47% of those offered $500, which the investigators say supports offering cash incentives to boost enrollment. The differences were significant (P < .001).
In contrast, the amount offered did not significantly incentivize more people to participate in the ambulation trial (P = .62). Rates were 45% with no compensation, 48% with $100 payment, and 43% with $300 payment.
In an analysis that adjusted for demographic differences, financial well-being, and Research Attitudes Questionnaire (RAQ-7) scores, each increase in cash incentive increased the odds of enrollment in the smoking cessation trial by 70% (adjusted odds ratio, 1.70; 95% confidence interval, 1.34-2.17).
The same effect was not seen in the ambulation trial, where each higher cash incentive did not make a significant difference (aOR, 0.88; 95% CI, 0.64-1.22).
“The ambulation trial was a lower-risk trial in which patients’ willingness to participate was higher in general. So there were likely fewer people whose participation decisions could be influenced by offers of money,” Dr. Halpern said.
Inducement vs. coercion
The incentives in the study “did not function as unjust inducements, as they were not preferentially motivating across groups with different income levels or financial well-being in either trial,” the researchers note.
Dr. Halpern and colleagues also checked for any perceptions of coercion. More than 70% of participants in each smoking cessation trial group perceived no coercion, as did more than 93% of participants in each ambulation trial group, according to scores on a modified Perceived Coercion Scale of the MacArthur Admission Experience Survey.
Furthermore, perception of risks did not significantly alter the association between cash incentives and enrollment in either trial.
After collecting the findings, Dr. Halpern and colleagues informed participants about their participation in RETAIN and explained the rationale for using different cash incentives. They also let all participants know they would ultimately receive the maximum incentive – either $500 or $300, depending on the trial.
Research implications
A study limitation was reliance on participant risk perception, as was an inability to measure perceived coercion among people who chose not to participant in the trials. Another potential limitation is that “neither of these parent trials posed particularly high risks. Future tests of incentives of different sizes, and in the context of higher-risk parent trials, including trials that test treatments of serious illnesses, are warranted,” the researchers note.
“While there are many more questions to ask and contexts in which to study the effects of incentives, this study calls on opponents of incentivizing research participants with money to be more humble,” Dr. Iltis said. “Incentives might not have the effects they assume they have and which they have long held make such incentives unethical.”
“I encourage researchers who are offering incentives to consider working with people doing ethics research to assess the effects of incentives in their studies,” Dr. Halpern said. “Real-world, as opposed to hypothetical studies that can improve our understanding of the impact of incentives can improve the ethical conduct of research over time.”