Table 3 Recruitment strategies with associated recruitment and retention rates of participants (n.d indicates that no data was available in the published literature)

Advertising

Crichton et al. 2012 [57]

35.7% of total participants (n = 30/84) via TV segment

31% of total participants (n = 26/84) via Newspaper

n = 20 males completed the study:

50% completers of 12-month follow up (n = 10/20)

(10% Early drop out (n = 2/20), Dropouts during study (40% n = 8/20)

Rose et al. 1976 [36]

n.d

11% attrition (n = 2,028/2,280). deceased n = 54 (2.4%); lost interest n = 103 (4.5%); moved away with no further participation n = 44 (1.9%); moved away with survey only participation n = 51 (2.2%)

Méjean et al. 2014 [43]

Initial recruitment strategies for NutriNet-Santé Study included television, radio, national and regional newspapers, posters and Internet providing details about the study’s website. A total of 86,652 individuals were recruited (n.d for individuals identifying as male)

n.d

Pogue et al. 2022 [54]

23.5% contacts made (n = 26,100/111,070) through social media. 58.6% views made from social media contact (n = 15,286/26,100). Recruitment rate of 73% for consent (1,987 attempts/1,452 consents were completed). 64.4% of participants were eligible (n = 1,279/1,452). Of these, 920 participants had demographic characteristics. Of those 3.5% (n = 32) were recruited via social media, of which 12.5% (n = 4) identified as male. Other media sources recruited a further 1.6% of participants (n = 15/920), of which 6.7% identified as male (n = 1/15). Word of mouth advertising recruited 11.2% (n = 103/920), of which 22.3% identified as male (n = 23/103)

63.4% attrition (n = 920/1,452) from consent to baseline sample where demographic characteristics were reported

Invitation Following online intake assessment at clinic, via telephone or via non for-profit platform

Van Wees et al. 2019 [46]

13,658 were eligible and invited. Overall, 12% of eligible individuals were recruited (n = 1,705), of these, 19% identified as male (n = 324)

From the total sample, 47.5% attended baseline visit (n = 810/1,705), 25.3% attended 3 week visit (n = 432/1,705), 24.3% attended 6 month follow-up (n = 416/1,705) and 20.2% attended 1 year follow-up (n = 344/1,705). Study attrition for both male and female participants by the 1 year follow up was 79.8%

Griffith Fillipo et al. 2022 [49]

1,652 were assessed for eligibility via the Mental Health America online platform. No details are given about how eligible participants were invited. 215 participants were randomized. Of these 14% (n = 30) identified as male

52.8 attrition rate for the low monetary incentive group and 54.1% attrition rate in the high monetary incentive group by exit survey at week 12, study end. While males were more likely to end participation early compared to females, no further information on the numbers are given

Letter of Invitation

Crichton et al. 2012 [57]

84 responders from advertising. Interested potential participants were invited to an information session and pre-study screening. 71 participants screened and deemed eligible (84.5%; n.d for individuals identifying as male)

Reported above

Cheraghi et al. 2021 [39]

All family members were invited for baseline measurements n = 15,005 individuals agreed to participate (≥ 3 years of age, n.d for individuals identifying as male)

39.6% of entire cohort after 5th follow up examination

n = 10,368 individuals participating in current study. Of these n = 4,395 (42.4%) were male (n = 1,650 for intervention, n = 2,745 for control)

Goldberg et al. 2006 [42]

GAZEL Study had 20,624 at baseline but 20,328 included in this study. 72.9% of participants identified as male (n = 14,827)

0.4% attrition rate over 12-year follow up due to leaving the company (n = 60), or leaving the study (n = 3)

87.2% of participants retained after first annual mail questionnaire returned in 1990 compared to baseline (n.d of numbers of participants or sex)

71.2% retained at the end of the study 12 years later in 2000 (n.d of numbers of participants or sex)

Green et al. 2018 [40]

Invitation letter signed by GP and information sheet describing aims, nature and how to contact Cam-CAN study was sent to 7,616 eligible individuals. 35.2% (n = 2,680) consented. Of those, 43.7% were male (n = 1,172)

n.d due to single time point reported in this study

Irvine et al. 2017 [41]

Letter received from GP inviting 47.1% of participants to take part in the study (n = 419 contacted out of 889 assessed for eligibility)

1.4% (n = 1 lost to follow up out of 69 consented participants)

Lee et al. 2009 [61]

Average recruitment rate of 40%. Recruitment Rate via city and registry: Florence (Primary care; 59.9% n = 433/723), Leuven (Electoral; 37.9% n = 451/1189), Lodz (City registry; 48.4% n = 408/843), Malmö (Population; 44.6% n = 409/918), Manchester (Primary care; 37.2% n = 396/1064), Santiago (National register; 35.2% n = 406/1155), Szeged (Electoral; 24.1% n = 431/1789), Tartu (Primary care; 59.2% n = 435/735)

Average attrition rate of 8.5%. Attrition rate calculated at each site due to death or moved house: Florence (4.4% n = 414/433), Leuven (4.4% n = 431/451), Lodz (15.7% n = 344/408), Malmö (11.3% n = 363/409), Manchester (11.1% n = 352/396), Santiago (21.2% n = 320/406), Szeged (0% n = unknown/431), Tartu (0% n = unknown/435)

Limmroth et al. 2023 [47]

Invitation sent as push messages to registered accounts for the app (n = 1,778). 79 patients consented (4.4%) of which 21 were male (1.18%). No gender specific data was given on the participants who withdrew or were excluded

0% attrition however, persistence (95%; n = 20/21 cf 90%; n = 19/21), compliance (primary end point) (97.7 n = 9/21 cf 95.4% n = 11.4/21) and adherence (90%; n = 19/21 cf 86% n = 18/21) were measured for each gender at 6 months compared to 12 months, respectively, from the prospective data set (above) and retrospective the data set: persistence (90%; n = 19/21 cf 86% n = 18/21), compliance (primary end point) (98.4% n = 6.7/21 cf 98.2% n = 4.3/21) and adherence (86% n = 18/21cf 86% n = 18/21)

Oleske et al. 2007 [52]

Eligible workers were sent letters from the study principle investigator, plant management and local union official inviting them to the study. N = 454 participants joined, of which 79.3% were male (n = 360)

Overall 31% attrition in 12 months (n = 141 drop outs), 83.6% of male participants had complete data (n = 168/201), 75.9% of male participants were considered drop out or had missing data collection (n = 192/253)

Ullman et al.1998 [56]

Mail surveys sent to participants with study information, a University pen and description of incentive once survey was sent back complete. Overall recruitment rate of 76% (n = 616/814; n.d for individuals identifying as male). 64.5% responded with no reminder (n = 334/518), 18.9% with one reminder (n = 98/518), 14.2% with two reminders (n = 74/518) and 2.3% with 3 reminders (n = 12/518)

32.6% of non-responders (n = 32/98) responded after receiving a second letter and $25 cheque

A substudy of males demonstrated that 93% were retained (201/216). Of these 46.3% of males responded with reminders only (100/216) and a further 7.9% responded with reminders and a cheque (17/216)

Overall attrition rate: 10.7% (n = 550/616)

Markanday et al. 2013 [58]

Letter of invitation contained information about the studies purpose, requirements of the participants and time investment required and where study centre was located. 67.1% recruitment rate for those who identified as male (n = 1,540/2,296), 77% for those identifying as female (n = 1,494/1,938)

n.d

Méjean et al. 2014 [43]

6,556 participated from a total of 15,000 randomly invited participants from the NutriNet-Santé Study. 6,352/6,556 participants available for analysis. Of this sample 1,531 male (24.1%)

From those who dropped out of the substudy (n = 9,982; defined as participating stopping within 6 months after the inclusion in the cohort), 2,398 identified as male (24%)

Purposeful Sampling

Amin et al. 2023 [38]

From 187 PREVENT-ADPKD trial participants completed the questionnaire 78% (n = 146/187). Of this sample

Saturation for post-trial interview was reached at 74% (n = 108/146). A further 18.4% declined an interview (n = 7/38). Groups not stratified by sex

Recruitment From Hospital or Research Centre

Crichton et al. 2012 [57]

16.7% of total participants (n = 14/84) indicated an interest for future studies at Research Centre. Written advertisement also placed at local hospital n.d on percentages recruited

Reported above

Gourash et al. 2022 [48]

n.d

2% (n = 50/2,458) attrition after year 1 assessment, 2.7% (n = 67/2,458) attrition after year 2, 3.5% (n = 80/2,458) attrition after year 3, 4.1% (n = 100/2,458) attrition after year 4, 4.9% (n = 121/2,458) attrition after year 5. Of which 0.4%, 0.7%, 1%, 1.2% and 1.7% was accounted by due to death, respectively. Groups not stratified by sex

Janus et al. 1997 [51]

n.d

7.2% attrition before the first-year assessment (n = 15/209) and 28.2% (n = 59/209) after the first-year assessment (assumed between years 1–4). 35.4% lost over the 4-year study (n = 74/209)

Kannisto et al. 2017 [44]

33.3% recruitment rate from eligible participants (n = 1,139/3,417). Of these, 49.2% (n = 560) who identify as male were randomised

4.8% attrition rate during the intervention period of all participants (n = 27/560) with 6 of the 27 withdrawals identifying as male (6/560; 1.1%). During a follow up postal survey 59% attrition rate who identify as male (n = 326/589)

Pogue et al. 2022 [54]

Recruitment rate reported above. 920 participants had demographic characteristics. Of these, 76.4% (n = 703/920) were recruited via Digital Health Journey, of which 26.2% (n = 184/703) identified as male. Another 1.6% were recruited through study referral at site (n = 15/920), of which 53.2% identified as male (n = 8/15)

Reported above

Schilling et al. 2023 [37]

99.2% of eligible individuals consented (n = 6,375/6,426). 92.63% (n = 5,905/6,375) of these individuals completed baseline surveys and of these participants, 74.02% identified as male (n = 4,371/5,905)

1.4% attrition rate from completion of baseline survey to COVID-19 analysis (n = 5,823/5,905). Groups not stratified by sex

Recruitment Through Community Venues – Town centre, Workplaces, Community Groups, Football Grounds, Golf Clubs, Library, Shopping Centre

Crichton et al. 2012 [57]

Written advertisements placed on noticed boards at libraries and shopping centres. n.d on percentages recruited

Reported above

Irvine et al. 2017 [41]

Field visits through community venues recruited the last 52.9% of participants (n = 470 males approached). One participant was recruited for every 11 community venue visits

Reported above