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Table 1 Characteristics of the included studies (grouped according to the relevant retention category)

From: Non-randomised evaluations of strategies to increase participant retention in randomised controlled trials: a systematic review

Trial Study design Host trial (number randomised) Disease/condition Participants Setting Retention strategies (number of participants) Outcome (retention study) Time point(s) used in the analysis
1. Studies that involved a change in mode of data collection
Atherton 2010 [25] Cohort study Prevention of Pelvic Infection (POPI) trial (1329) Chlamydia screening Young female students 12 universities and colleges across London A postal follow-up questionnaire (299)
Online questionnaire (1030)
Questionnaire response rate 12 months after randomisation
4 weeks and 12 months after follow-up commenced
Childs 2015 [26] Cohort study Prevention of Lower Back Pain in the Military (POLM) trial (4325) Low back pain Geographically dispersed soldiers in the US Army A military training setting Web-based survey (632)
Telephone call center (571)
Both the web-based
and telephone survey (233)
M2 database (2788)
Follow-up rate Monthly follow-up surveys (12 weeks after training).
Telephone contact with soldiers who had not responded to 3 monthly web-based surveys at the end of the first year
Dormandy 2008 [27] Before and after study SHIFT (Screening for Haemoglobinopathies in the First trimester) (775) Antenatal Sickle Cell and Thalassaemia (SCT) screening People from minority ethnic groups and with high levels of material and social deprivation UK primary care Postal questionnaire completion only (61)
Choice of telephone or postal completion (714)
Questionnaire response rate 11 months after randomisation
Johnson 2015 [28] Before and after study The hospital outpatient alcohol project (HOAP) (837) Alcohol consumption Hospital outpatients with hazardous or harmful drinking Outpatient department of a large tertiary referral hospital in Newcastle (population 540,000), Australia Postal questionnaire + link a web-based questionnaire (520/837)
Telephone follow-up (317)
Questionnaire response rate 6 months after randomisation
4 weeks later if questionnaire remained unreturned
Lall 2012 [29] Cohort study Back Skills Training Trial (BeST) (701) Back pain Patients with subacute and chronic low back pain UK primary care Postal questionnaires
Telephone interviews
Questionnaire response rate 12 months after randomisation
6 weeks later if questionnaire remained unreturned
Peterson 2012 [30] Post hoc analysis method Randomised clinical trial of elective coronary artery bypass grafting (248) Coronary artery disease Coronary artery bypass graft surgery patients The New York Hospital–Cornell Medical Center Routine follow-up approach (return to hospital for follow-up) (187)
Home follow-up (61)
Follow-up rate and its impact on main trial outcome 6-month follow-up
2. Studies that used a different questionnaire format for follow-up
Bailey 2013 [31] Before and after study The Sexunzipped trial (2006) Sexual well-being Young people aged 16 to 20 years and resident in the UK Online study Online questionnaire (1208)
A shortened version of the online questionnaire by post (798)
Retention of valid participants at 3-month follow-up 3 months after randomisation
3. Studies that used a different design strategy for follow-up
Ulmer 2008 [32] Historically controlled study Randomised controlled trial of a telephone-delivered behavioral intervention (153) Hypertension Participants with uncontrolled hypertension New York Harbor Healthcare System A 4-week simple run-in period before participation in the study Drop-out rate 12 months after randomisation
4. Studies that used a change in mode of reminder delivery
Hansen 2014 [33] Cohort study ‘Preventive consultations for 20- to 40-year-old young adults’ randomised trial (495) Change in health behavior Young adults with multiple psychosocial and lifestyle problems General practices in Denmark Follow-up questionnaire and up to two reminders by mail (495)
Telephone reminder to primary non-responders (179)
Questionnaire response rate 1-year follow-up after randomisation
Varner 2017 [34] Nested cohort analysis An RCT assessing minor traumatic brain injury (MTBI) discharge instructions (118) Traumatic brain injury Patients ages 18 to 64 years presenting to the ED with a chief complaint of a head injury or suspected concussion Emergency department (ED) of an academic tertiary care hospital in Toronto, Ontario Telephone contact (78)
A reminder text message (40)
The proportional difference in loss to a follow-up between the two groups First 4 months
Final 3 months
5. Studies that used incentives
Brealey 2007 [35] Historically controlled study DAMASK Trial (a pragmatic randomised trial to evaluate whether GPs should have direct access to MRI for patients with suspected internal derangement of the knee) (547) Knee problems Patients aged 18 to 55 with suspected internal derangement of the knee General practices across North Wales, North East Scotland, and Yorkshire No incentive (105)
Unconditional direct payment of £5 to patients for the completion and return of questionnaires (442)
Completion rate of postal questionnaires 12 months after randomisation
Rodgers 2016 [36] Prospective cohort study An RCT evaluated the effectiveness of a brief social work intervention (479) Instances of violence and heavy drinking among women Abused women who were also heavy drinkers Two US academic urban EDs Cash incentives for participants enrolled during the first 8 months of the study (111)
A reloadable bank card for all subsequent participants (358)
Participant completion rates of follow-up study activities and overall retention 3-, 6- and 12-month follow-up after randomisation
6. Studies that used multi-faceted strategies
Ezell 2013 [37] Post hoc analysis method RCT comparing asthma outcomes of students exposed to tailored asthma management versus those exposed to generic asthma management (422) The burden of asthma Students in grades 9 through 12 Six Detroit public high schools Incentives ($80 for completion of all program modules) (380).
4 retention strategies to locate missing participants (re-dials of non-working telephone numbers, Facebook, assistance from school) (125)
Attrition rate 12-month follow-up
Sellers 2015 [38] Before and after study with no control group The BAN trial was designed to evaluate the efficacy of 3 mother-to-child HIV transmission prevention strategies (2369) HIV prevention Pregnant women who tested HIV-positive and their infants Four antenatal clinics Lilongwe, Malawi Routine strategies (support groups, home visits) (1686)
Intensive tracing efforts (638)
Attrition rate 28 weeks after randomisation