But in general, the variability in the data used and its relevance to the central question of infrastructure’s impact on growth, the model specification, the econometric methodology and the treatment of non-stationarity and causation, are all causes for inconclusive results. These data difficulties also make it hard to arrive at methodologies to compare and contrast the experiences of countries in promoting growth through increases in stock in infrastructure. Straub (2008: 22) reviewed 64 empirical studies linking infrastructure to growth to find that veiy few of them actually addressed the question directly and systematically. Values are averages for each decade, based on banks' annual commitments from both concessional and nonconcessional windows. World Bank: includes International Bank for Reconstruction and Development and International Development Association only. Inter-American Development Bank: the 1960s are the average over 1967-1969.