A thank-you card from the research team sounds like an easy way to keep clinical trial volunteers engaged. In a study published in May 2026, it made no clear difference in whether 560 participants returned their questionnaires.
That result adds to a growing pile of evidence that friendly mail is not the answer. In December 2019, researchers sent a snowy winter card reading "Season's Greetings" to 1,617 people enrolled in clinical trials across the U.K., while another 1,606 got nothing. When their next follow-up came due, the two groups looked almost identical: 85.3% of analyzed card recipients completed it, compared with 85.4% of those who received no card.
