Postal study invitations
CHILDHOOD CANCER, RECRUITMENT
Personal identification numbers of children who had completed treatment 3 months to 5 years previously were provided by the Swedish Childhood Cancer Registry (CCR), and linked to parents' names and addresses via NAVET, a population registry from the Swedish Tax Agency. The first recruitment block was pre‐selected with parents of children who had ended treatment near to 5 years previously. The following four blocks were randomly selected by a member of the Portal team, independent to the research team, using a computer‐generated simple randomization procedure. Postal study invitation packs, sent to parents' home addresses, included a: (1) study invitation letter; (2) study information sheet and link to a secure website on the Portal (information in text and video format); (3) paper reply‐slip to register interest in participation; (4) paper‐based opt‐out form and reasons for non‐participation questionnaire; and (5) freepost envelope. Parents could register interest in participation and request more study information via the Portal, post, telephone or e‐mail. ENGAGE included an embedded recruitment RCT, investigating the effect of personalized versus non‐personalized study invitations on recruitment and retention