Table 8 Summary of the results from ‘ retention ’ studies included in the review (n = 26 studies)

Difficulty maintaining contact; highly mobile populations; frequently changing contact numbers

• Implementing tracking procedures with multiple contact methods – i.e., phone, mail, email [17, 20, 22, 30, 39, 40, 54, 66],[74, 75, 81, 85].

• Contact details of significant others [20, 30, 76, 66].

Practical barriers such as transport difficulties, lack of child care, lack of leave from work

• Incentives (cash and other gifts) to study participants as reimbursement for time [18, 22, 23]*,[24]*,[30, 39, 40, 70–73, 76, 80, 81, 123].

• Participant-centred approach: personalised, tailored individualised approach to follow-up calls or visits [39, 54, 66, 73, 96, 123] and flexible (accommodating) protocols [16, 30, 35, 66, 70, 72, 74, 76] and providing transport or child care [30].

• Scheduling follow-up assessments to coincide with existing appointments [22, 30].

• Toll-free numbers [39, 54].

Forgetting to return for follow-up

• Use of study logos on gifts [74].

• Phone text message reminders [65].

• Keep in regular contact [18, 22, 30, 65, 67, 76, 81, 118].

• Highlighting benefits of research during follow-up contact [30, 40, 71, 72].