Enhancing reach and effectiveness of recruitment through email: a study within a trial (SWAT)

Best practices for efficient and effective email recruitment are unclear. This study within a trial (SWAT) aimed to evaluate the effect of an email's subject line and a prenotification email, as well as explore the impact of sociodemographic characteristics, on recruitment.

The SWAT was embedded within the Buddy Check Postcard Project (BCPP) trial, which investigated sending brief caring postcards to older veterans to reduce loneliness. The SWAT was a quasi-experimental study (n = 1945) with time-based allocation that tested three subject lines and use of a prenotification (email notifying participants to expect a study invitation). Subject lines each emphasized a different potential motivator for research participation: benefit to self (incentive), benefit to others (altruism), and official Department of Veterans Affairs (VA) email (institutional reference). The primary outcome was reach (whether recipients opened the email) and the secondary outcome was effectiveness (whether recipients completed the baseline survey). Sociodemographic characteristics considered were age, sex assigned at birth, race, ethnicity, and rurality.

The institutional subject line was associated with higher odds of opening the email compared to the altruism (adjusted odds ratio (AOR) = 1.93, 95% confidence interval (CI): [1.46, 2.55]) and incentive subject lines (AOR = 1.54, 95% CI: [1.19, 1.99]), adjusting for sociodemographic characteristics, site, and prenotification. The institutional subject line was also associated with higher odds of baseline survey completion than the altruism subject line (AOR = 2.44, 95% CI: [1.08, 5.50]), but not compared to the incentive subject line (AOR = 1.42, 95% CI: [0.73, 2.76]). Receiving a prenotification email was not significantly associated with increased reach or effectiveness. Non-Hispanic/Latino White urban participants were more likely to open the recruitment email. Younger, female, non-Hispanic/Latino White participants were more likely to complete the baseline survey. There was no evidence of sociodemographic characteristics moderating the effects of either the subject line or prenotification.

Veterans who received an email subject line with an institutional reference to the VA were more likely to open the email and enroll in a clinical trial than veterans who received an incentive or altruistic subject line. This study did not detect clear evidence of a substantial benefit for recruitment to sending prenotification emails.