Tax expenditures are defined as deviations from a country-specific standard or benchmark tax system. The GTED uses data published by governmental institutions, sticking to their own definitions of benchmarks. It does not complement official figures or challenge what countries consider as their benchmark tax system. Without universally recognized standards for tax benchmarking, cross-country comparisons should be interpreted with caution. Other issues that potentially affect the comparative analysis of GTED data are methodological changes in government estimates of revenue forgone, as well as changes in benchmark definitions. When governments do not document these changes in their own reports and adjust the respective time series, data consistency over time can be limited. Additionally, while adding up revenue forgone estimates of individual measures can be helpful to indicate the overall size of tax expenditures, the sum of individual estimates does not necessarily equal the total due to behavioral responses and interactions across provisions.