Barber, S., Jackson, C., Akhtar, S., Bingham, D., Ainsworth, H., Hewitt, C., …Wright, J. (2013). Pre-schoolers in the playground: an outdoor physical activity intervention for children aged 18 months to 4 years old: study protocol for a pilot cluster randomised controlled trial. Trials, 14(1), Article 326. https://doi.org/10.1186/1745-6215-14-326