Stephenson, J., Strange, V., Forrest, S., Oakley, A., Copas, A., Allen, E., …team, &. T. R. S. (2004). Pupil-led sex education in England (RIPPLE study) : cluster-randomised intervention trial. The Lancet, 364(9431), 338-346. https://doi.org/10.1016/s0140-6736%2804%2916722-6