Quantitative evaluation of recruitment strategies in a cluster-randomized trial: segmented regression and cost
Nicola Litke1, Michel Wensing1, Lina Hermeling2
1Department of General Practice and Health Services Research, University Hospital Heidelberg, Im Neuenheimer Feld 130.3, Heidelberg, 69120, Germany.
Background:
Recruitment of participants for preventive health intervention studies remains a significant challenge: Approximately 19% of studies are discontinued due to insufficient participant numbers, and one in three extends its recruitment period. This study aimed to examine the impact of recruiting measures on the recruitment rate over time and to describe costs of these strategies to inform future study planning and optimize resource allocation.
Methods:
A cluster-randomized controlled trial ("AOK-Family+" study) was conducted from April 2023 to June 2024 in southwestern Germany. The intervention focused on reducing lifestyle-related risk factors (LRRFs) among pregnant women and women planning a pregnancy. Analog recruitment strategies included e.g., printed magazines, flyers and digital recruitment strategies included e.g. social media ads, influencer marketing. For 255 interested women, contact details, recruitment source, and participation status were documented and analyzed. A segmented linear regression analysis was used to identify turning points in application trends. Direct costs were calculated based on internal project budget tracking.
Results:
The targeted sample size was not reached despite substantial investment in recruitment measures. The highest number of applications resulted from analog strategies-especially printed AOK magazines (35.5%)-followed by influencer marketing (23.6%). The segmented linear regression analysis identified three significant increases in application rates, the first coinciding with the magazine distribution and the second with the influencer marketing on Instagram. Social media marketing showed a short-lived effect, with application rates dropping immediately after posts ended. Total costs amounted to 99.334 € equaling 389,54€ per application and 534,05€ per actually enrolled participant.
Conclusion:
Health magazines proved to be the most cost-efficient and sustainable recruitment strategy. Influencer marketing led to high reach and initial spikes in engagement but had limited long-term impact. While digital measures generated many clicks, only a small fraction translated into study participation-indicating a pronounced intention-behavior gap and procedural barriers in the enrolment process. For future studies, a mixed-methods recruitment strategy is recommended combining wide digital outreach with personalized, trust-based communication, ideally through healthcare professionals such as gynecologists and midwives, to reach women in early pregnancy and reduce participation barriers.
Trial Registration:
The German Clinical Trials Register DRKS00027804. Registered on 2022/01/12.
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