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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