This case describes a current evaluation of an educational intervention to help disadvantaged children catch up in literacy at around the time they transfer to their senior school. The evaluation is based on a randomised controlled trial design, which the case explains is the most appropriate for testing or establishing a causal claim. The case also demonstrates how necessary, how ethical and how easy it is to conduct such a trial. The trial naturally includes process evaluation, using in-depth approaches such as observation and interviews to assess how well the intervention is being conducted in practice. A crucial point is that the evaluators must have no vested interest in the success or failure of the intervention.