Eight included retention trials were embedded in trials for the treatment of alcohol and smoking dependency (Bauer 2004ab; Hughes 1989; Khadjesari 2011 1abc; Khadjesari 2011 2; McCambridge 2011 1; McCambridge 2011 2; Severi 2011 1; Severi 2011 2), and four in trials investigating treatments for injuries (Edwards 2001; Gates 2009; Nakash 2007; Svoboda 2001).
For the embedded retention trial, we extracted data for onset in relation to the host trial, source of the sample, aim, primary outcome and type of follow-up.
The retention trials were embedded in real trials (host trials) and not hypothetical trials.
The sample size of the CAP trial was determined by the number of practices required to meet the recruitment target for the embedded ProtecT trial.
Be it external CROs, in-house pharmaceutical monitors or quality management teams embedded at trial sites.
The feasibility study tested the most reliable means of collecting these data through an embedded randomised trial comparing simple prospective diaries and retrospective questions within the main outcome questionnaires.
In all analyses (including the bivariate analyses), we included a variable for survey time point (6, 12, 18, 24, 30, or 36 months) and a variable for intervention arm to account for any potential design effects of the embedded randomized trial.
The embedded ProtecT trial (Donovan et al, 2003; Lane et al, 2010) (ISRCTN20141297) compares the effectiveness and cost-effectiveness of radical prostatectomy, radical conformal radiotherapy and active monitoring (regular PSA testing and treatment review) in men with PSA-detected clinically localised prostate cancer.
Research embedded within trials will also help to explore whether parental responses to recruitment vary depending on how well their child recovers.
Seven retention trials were embedded in prevention trials, which included two cancer prevention trials for lung and breast cancer (Bowen 2000abc; Sutherland 1996), one migraine prevention trial (Ashby 2011), and three fracture prevention trials (Avenell 2004; Cockayne 2005; MacLennan).
