1. A Cochrane risk of bias assessment tool: for non-randomized studies of interventions (ACROBAT-NRSI);Sterne,2014
2. Wells G, Shea B, O'Connell D, et al. The Newcastle-Ottawa Scale (NOS) for assessing the quality of nonrandomised studies in meta-analysis. Available at: http://www.ohri.ca/programs/clinical_epidemiology/oxford.asp. Accessed October 22, 2014.
3. Systematic review identifies number of strategies important for retaining study participants;Robinson;J Clin Epidemiol,2007
4. A systematic review of the effect of retention methods in population-based cohort studies;Booker;BMC Public Health,2011
5. Strategies to improve retention in randomised trials;Brueton;Cochrane Database Syst Rev,2013