Bray, I., Noble, S., Boyd, A., Brown, L., Hayes, P., Malcolm, J., …Tilling, K. (2015). A randomised controlled trial comparing opt-in and opt-out home visits for tracing lost participants in a prospective birth cohort study. BMC Medical Research Methodology, 15(1), https://doi.org/10.1186/s12874-015-0041-y