Data from Transparency International’s Global Corruption Barometer (GCB) is useful for diagnosing corruption. Development practitioners can in particular benefit from the tool at the early identification and formulation stages of a programme or a strategy, by making inferences regarding the state of corruption in a country and sector institutions. It is preferable to many other indices because it provides experience-based data. However, if questions on attribution, effectiveness or impact of specific reforms or organisations are asked, then more intervention-specific data is needed. The GCB can only be used as supplementary data when appraising, monitoring and evaluating policies, programmes, and institutions.