Coleman E, Arundel C, Clark L, Doherty L, Gillies K, Hewitt C, Innes K, Parker A, Torgerson D, Treweek S (2021)

Bah humbug! Association between sending Christmas cards to trial participants and trial retention: randomised study within a trial conducted simultaneously across eight host trials. BMJ 375, e067742.
Meta-analysis of a series of Studies Within a Trial (SWAT) testing the impact of Christmas cards on retention in randomised trials.

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