Finally, it is important to state that the evidence presented in this review - which is based exclusively on quantitative analyses - is produced by adopting different methodological approaches, ranging from studies based on face-to-face interviews or focus groups with relevant stakeholders to more sophisticated econometric analyses. The latter approach lead to issues linked to the reliability of comparing findings based on simulations or on before-and-after analyses which often fail to capture the casual relationship between the programme and a given outcome with experimental approaches, such as randomized trials, which can give a more realistic estimation of the impact of a programme. Besides the intrinsic limitations of this kind of analysis, heterogeneity in the quality and extent of existing empirical methods makes comparison across similar instruments in different contexts, or over time, difficult, undermining the extent to which we can extract normative conclusions from this study.