This article describes the contribution of mixed methods using a study that had a two-fold objective: designing an intervention programme to promote family reading activities, specifically for socially disadvantaged families; and proving existence of a causal link between the implementation of this programme and improvements in the vocabulary of the children enroled in the programme. While conducting a randomized controlled trial, generally associated with quantitative methods, this study integrated a qualitative component both upstream and downstream. Upstream, the study sought to refine knowledge about the conditions of implementation and reception of the programme using “instrumental qualitative“ methods (the strategy of “systematization“). Downstream, an ethnographic survey allowed for a better understanding of the processes that led to the measured impacts, using an “empowered qualitative“ approach (the “enrichment“ strategy). After presenting the research strategy adopted with reference to the so-called realistic approach to evaluation, this article will discuss the importance of mixed methods by highlighting the two types of contributions that qualitative field studies can make to randomized experiments.