Some countries fail to ensure that their citizens and businesses make an appropriate contribution to the financing of public tasks. But not all countries with a low tax ratio automatically fall into this category. This paper presents an approach to bridge the gap between probabilistic statements based on statistical analyses, and country-specific information. Rather than defining general across-the-board criteria, the approach accounts for different development levels and other influencing factors, such as regional patterns, non-tax revenue and governance. Findings on individual countries or groups of countries should put governments, donors and international organisations in a better position to decide on tax reform programmes and aid modalities.