Table 1.

Description of recruitment sources for the INTERVAL trial

Recruitment source	Source description	Invitation period	Invitation type
Centre	Donors registered at a static donor centre	Jun 2012 – Jan 2013	Letter
Mobile (platelet)	Mobile session donors who had previously registered an interest in giving platelets at a static donor centre	Jan 2013 – Oct 2013	Letter
Mobile (10-mile)	Donors giving blood at a mobile session within 10 miles of a static donor centre	Jan 2013 – Oct 2013	Letter or email
Mobile (30-mile)	Donors with a correspondence address within 30 miles (but typically 20 miles) of a static donor centre	Jan 2013 – Oct 2013	Email
No invitation	Donors giving blood at a static donor centre who expressed an interest in joining the trial but who did not receive a previous invitation	Jun 2012 – Jun 2014	Not applicable