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Introduction: Recruitment into clinical trials is a common challenge experienced by healthcare researchers. Currently, there is little evidence regarding strategies to improve recruitment into clinical trials. However, preliminary research suggests the personalisation of study invitation letters may increase recruitment rates. As such, there is a need to investigate the effectiveness of personalisation strategies on trial recruitment rates. This study within a trial (SWAT) will investigate the effect of personalised versus non-personalised study invitation letters on recruitment rates into the host trial ENGAGE, a feasibility study of an internet-administered, guided, Cognitive-Behavioural Therapy (CBT) based self-help intervention for parents of children previously treated for cancer.

Methods: An embedded randomised controlled trial (RCT) will investigate the effectiveness of a personalised study invitation letter including the potential participant's name and address compared with a standard, non-personalised letter without name or address, on participant recruitment rates into the ENGAGE study. The primary outcome is differences in the proportion of participants recruited, examined using logistic regression. Results will be reported as adjusted odds ratios with 95% confidence intervals.

Discussion: Even moderate effects of the personalisation of study invitation letters on recruitment rates could be of significant value by shortening study length, saving resources, and providing a faster answer to the clinical question posed by the study. This protocol can be used as a template for other researchers who wish to contribute to the evidence base for trial decision-making, by embedding a similar SWAT into their trial.

Trial registration: ISRCTN 57233429; ISRCTN 18404129; SWAT 112, Northern Ireland Hub for Trials Methodology Research SWAT repository (2018 OCT 1 1231).