Opt‐out and reminders
Parents invited via the post could opt‐out of ENGAGE via the Portal, post, telephone, or e‐mail. Up to five reminder telephone contact attempts were made if parents did not respond within 4 weeks of invitation. Telephone numbers were identified using internet search engines. Contact attempts were documented in paper‐based case report forms (CRFs). If a telephone number was not identified, a postal study invitation reminder letter was sent.