A 2011 study argues that previous assumptions that oil abundance is a curse were based on methodologies which failed to take into account cross-country differences and dependencies arising from global shocks, such as changes in technology and the price of oil. The researchers studied data from the World Bank over the period 1980–2006 for 53 countries, covering 85% of world GDP and 81% of world proven oil reserves. They found that oil abundance positively affected both short-term growth and long-term income levels. In a companion paper, using data on 118 countries over the period 1970–2007, they show that it is the volatility in commodity prices, rather than abundance per se, that drives the resource curse paradox.