JMIR Publications

ABSTRACT

Background:

e-cohorts are susceptible to low participation rates, undermining representativeness. Frequent reminders can be a cost-effective strategy to increase response rate. However, their effectiveness may vary depending on the delivery mode, the content, and the formatting of messages.

Objective:

To compare different reminder strategies (i.e., emails and/or text messages with standard and/or institutional formatting) and evaluate their effectiveness in terms of response rate, in the context of a population-based e-cohort of healthy adults.

Methods:

We conducted a 4-arm randomized-controlled trial nested in Le French Gut e-cohort (registration number 2021-A01439-32). In November 2024, we included participants who were enrolled online (SKEZIA plateform) but not yet active participants (i.e., eligibility, consent form, and/or personal information questionnaire not completed). Randomization was stratified on time since enrolment. We sent three reminders, 72-h apart, following four experimental designs: (Group 1) standard emails only; (Group 2) text messages only; (Group 3) institutional emails only; and (Group 4) standard email, followed by text message and institutional email. Our primary outcome was the completion rate of the personal information questionnaire. We also measured the completion time (i.e. time between reminder received and questionnaire fully completed), online login rate, login time, email opening, and click-through rates.

Results:

At the end of the trial, out of 20,487 eligible participants, 19,525 received at least one reminder. The per-protocol completion rate was 8.4%, with a higher rate in Group 4 (9.4% vs 7.4%, 8.4% and 8.3% for Groups 1, 2, and 3, respectively; P <.001). Completion time was faster in Group 2 (mean ± SD, 6.3 ± 4.3 days) compared to Groups 3 and 4 (7.0 ± 3.5 and 7.2 ± 3.8 days; P = .003). Online login rate was higher and login time faster in Group 4 (rate: 15.6% vs 12.1%, 14.3% and 15.3% for Groups 1, 2, and 3, respectively; P <.001; and time: 7.1 ± 4.3 days vs 6.7 ± 4.3, 6.2 ± 4.6, 6.8 ± 3.9 for Groups 1, 2, and 3, respectively; P = .002). For emails, opening rates were similar (P = .87) but click-through rate was higher for institutional emails (23.1% vs 19.6% for standard formatting; P <.001).

Conclusions:

A mixed-delivery mode strategy, combining emails and text messages, effectively increases response rate by 27% compared to other strategies. Institutional emails with plain design and signed by study coordinators, seemed more appealing to participants than a more elaborate design. Clinical Trial: registration number 2021-A01439-32