The artificial nature of the target of debt to GDP of 120% for Greece in 2020. This is the target being followed in negotiations by the troika of the ECB, IMF and the EU. Experts say the sustainable level would be much lower for Greece -this would be much lower because of the aging population in Greece and lower level of workers to support retirees in future years, the inefficient tax collection system and poor prospects for changing it, the degree of control over monetary policy and the rate of change of debt. A recent study by the Bank for International Settlements shows debt sustainability at 85% after studying 18 countries from 1980 to 2010. No precise source has been found for the 120% target. An IMF Report in 2011 said the 120% was the "maximum level considered sustainable." Alan Auerbach at UC Berkeley and Michael Woodford at Columbia University, say the additional factors are relevant to Greece. The many unpredictables over the course of ten years is another serious difficulty.