Background
Recruitment for clinical trials is known to be challenging, particularly in mental health where stigma hinders enrolment. Social media offers a promising, costs-efficient alternative to increase reach and diversity. This study evaluated the effectiveness of theory-based behavioural advertisements strategies to recruit participants for a Dutch randomized controlled trial on mobile depression screening.
Methods
A six-month recruitment campaign was launched in the Rotterdam (Netherlands) region using Meta Ads including three 'flights'. Each flight tested advertisement variants based on six behavioural strategies with two different stock photos: autonomy ('choice is yours'), social importance('help improve mental health for all'), social proof('hundreds have already joined'), commitment('I want to help others'), scarcity('sign up today, registration closes soon'), and altercasting('you can lead early detection of depression'). Campaign performances were evaluated through impression, clicks, cost-per-click(CPC), click-through rate(CTR), and participant enrolment. Continuous A/B testing enabled systematic comparison and optimization across flights.
Results
The campaign generated over 600,000 impressions and 8,795 clicks across all flights. Autonomy-themed advertisements consistently outperformed others, achieving the highest CTR (1.83%) and clicks (n=2,650) in Flight I with variant a (two persons in the Rotterdam subway). In Flight II, autonomy again outperformed all other strategies, with variant b (two men in front of the Erasmus Bridge) generating more clicks(n=1,448) and a higher CTR(1.73%) compared to variant a (two persons in the Rotterdam subway; n=719 clicks,CTR 1.56%). Scarcity messages (variant a and b) showed moderate average effectiveness (n=702 clicks, CTR 1.49%), while social importance, social proof, commitment, and altercasting remained consistently less engaging across all flights(<1.3% CTR). Overall, CPC were stable across flights(€0.25-0.31). Of the total reach, approximately 500 individuals registered via the study website, of whom about 260 proceeded to full RCT enrolment by providing informed consent and completing the baseline assessment. When including all costs (campaign, management, and implementation costs), average cost per enrolled participant was €7.50.