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IMF's forecast said that Greece's unemployment rate would hit the highest 14.8 percent in 2012 and decrease to 14.1 in 2014.  But in fact, the Greek economy suffered a prolonged high unemployemnt. The unemployment figure was between 9 per cent and 11 per cent in 2009, and it soared to 28 per cent in 2013. In 2015, Greece's jobless rate is around 24 per cent. It is thought that Greece's potential output has been eroded by this prolonged massive unemployment due to the associated hysteresis effects.

What is thought to have eroded Greece's potential output?
prolonged massive unemployment