Randomized control trial of unconditional versus conditional incentives to increase study enrollment rates in participants at increased risk of lung cancer.

Introduction: Understanding how incentives and their timing influence study enrollment rates is important to efficient study design and increasing the generalizability of findings. This 2-arm, parallel randomized trial evaluated how conditional vs. unconditional mailed incentives of a $20 gift card affected study enrollment in a sample of participants screened for lung cancer screening.

Methods: Eligible participants included Black and White adults who underwent lung cancer screening with low-dose CT and had negative screening results at two North Carolina imaging facilities in 2018. We used a stratified randomization scheme, by sex and race, to assign incentive type (conditional vs. unconditional). We used the Tailored Design Method with six points of mailed contact to engage participants. We compared study enrollment rates using chi-square tests and logistic regression analyses.

Results: After adjusting for sex, race, age, smoking status, participant residence, and screening site, participants who received unconditional incentives were 74% more likely to enroll than those who received conditional incentives (adjusted OR = 1.74 (95% CI: 1.01, 3.00).

Conclusions: Type of incentive can play a role in increasing study enrollment, especially mailed surveys that target individuals who currently or previously smoked. Unconditional incentives may be worth the initial cost to engage study participants.