Adding 'Withdrawal Slips' within follow up reminders for questionnaires

To evaluate the effects on retention, questionnaire completion and trial efficiency of including withdrawal slips in follow-up reminders for questionnaires in a randomised trial, which allow
participants to ‘skip’ that timepoint but remain in the study for further follow-up or to ‘stop’ any further follow-up questionnaires (i.e. to ‘withdraw’ from the study).