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Table 2 Data provided by additional RCTs identified through contact with CTUs and trial methodologists for a systematic review of strategies to improve retention in randomised trials

From: Identifying additional studies for a systematic review of retention strategies in randomised controlled trials: making contact with trials units and trial methodologists

Eligible RCT

RCT comparison

Number of participants results are based on

Meta-analysis of incentive strategies

 Cockayne (2005) [9]

Addition of offer of non-monetary incentive versus no offer

1038

 Brown (1997) [8]

Entry into a prize draw versus no entry into a prize draw

No data available

 Gates (2009) [14]

Monetary incentive versus no incentive

2144

 Khadajesari (2011) (a) [15]

Addition of offer of monetary incentive/ entry into prize draw/monetary donation to charity with reminder email versus no offer with reminder email

1837

 Khadajesari (2011) (b) [15]

Addition of offer of monetary incentive with reminder email versus no offer with reminder email

2591

 Bailey 1

Addition of £20 voucher versus addition of £10 voucher

417

 Bailey 2

Addition of £10 voucher plus offer of £10 voucher versus addition of £5 plus offer of £5

485

Total

8521

Meta-analysis of communication strategies

 Marson (2007) [12]

Enhanced letter versus standard letter

1815

 Nakash 2007 [11]

Additional reminder versus usual follow-up procedures

298

 Ashby 2011 [10]

Additional reminder versus usual follow-up procedures

148

 MacLennan [13]

Additional reminder versus usual follow-up procedures

753

 Mitchell

Pre contact and participant up-date via newsletters

No data available

 Severi (2011) (a) [16]

Text message reminder plus a fridge magnet versus standard follow-up

1950

 Severi (b) (2011) [16]

Reminder telephone call from principal investigator versus standard follow-up

127

Total

5091