It was chosen by the NEA as the basis for its SSDI (Simplified Supply and Demand Index) which was presented in Chapter 3. On the basis of a complete set of energy data provided by the International Energy Agency’s Data Services from 1970 to 2008, the SSDI tracked energy supply security based on both supply and demand data. It showed that the security of supply situation in OECD countries has unequivocally improved since the early 1970s.