SMS text pre-notification and delivery of reminder e-mails to increase response rates to postal questionnaires in the SUSPEND trial: a factorial design, randomised controlled trial

Background: Patient-reported outcomes are vital in informing randomised controlled trials (RCTs) and health-care interventions and policies from the patient's perspective. However, participant non-response may introduce bias and can affect the generalisability of the trial. This study evaluates two interventions aimed at increasing response rates to postal questionnaires within a large, UK-wide RCT: pre-notification via short messenger service (SMS) text prior to sending the initial mailing of trial questionnaires versus no pre-notification; for non-responders to the initial mailing of the questionnaires, an e-mail reminder (containing a hyperlink to complete the questionnaire online) versus a postal reminder.

Methods: This study is a 2x2 partial factorial design RCT nested within an RCT of medical expulsive therapy for ureteric stone disease. Participants who supplied a mobile telephone number were randomly assigned to receive an SMS text pre-notification of questionnaire delivery or no pre-notification. Those who supplied an e-mail address were randomly assigned to receive a questionnaire reminder by e-mail or post. Participants could be randomly assigned to the pre-notification comparison or the reminder comparison or both. The primary outcome measure was response rate at each questionnaire time point.

Results: Four hundred eighteen participants were randomly assigned to the SMS pre-notification comparison (80 % were male, and the mean age was 41 years with a standard deviation (SD) of 11.1). The intervention had no effect on response rate at either questionnaire time point. In subgroup analyses, SMS pre-notification increased response rates in women but only at the first questionnaire time point. One hundred nineteen participants were randomly assigned to the reminder comparison (80 % were male, and the mean age was 42 years with an SD of 12.1). There was no difference in response rate in those who received an e-mail reminder compared with those who received a postal reminder.

Conclusions: SMS text pre-notification of questionnaire delivery and email delivery of questionnaire reminders did not improve response rates. There was some evidence to suggest that SMS text pre-notification may be effective in women, and further studies to investigate this may be warranted. E-mail reminders for participants to return their postal questionnaire could be advantageous given that response rates were similar following either type of reminder and the low cost of delivering an e-mail compared with a postal reminder.