Qualitative Comparative Analysis (QCA) is a comparative research methodology devised by Charles Ragin in 1987 with the aim of analysing small samples. It is part of the Hybrid Methods and offers a strategy that lies somewhere between qualitative case analysis and the quantitative study of variables. QCA aims to achieve two apparently contradictory objectives: on the one hand, to gather detailed information to provide in-depth knowledge of the cases studied; on the other hand, to produce results that are generalisable. QCA embodies the best of case studies in the sense that each case is considered as a configuration of conditions and outcomes, none of which are left out in the analysis. In parallel, it develops a concept of causality known as multiple conjunctural causality, according to which: i) it may be a specific combination of conditions that generates a certain phenomenon (the outcome); ii) different combinations of conditions may lead to the same outcome; iii) depending on the context, the same condition may have different impacts on the outcome. The approach involves a systematic and rigorous comparison of selected cases using Boolean logic (variables can only take on the values true or false) and an analytical protocol based on data analysis software, such as R or STATA, to ensure a thorough examination of different configurations. It is considered appropriate for the analysis of ex-post research designs, which question the factors that cause a given phenomenon, such as the outcomes of the legislative process, public spending, coalition formation, policy change or the success of political projects