The Natural rate of unemployment reference article from the English Wikipedia on 24-Apr-2004
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Natural rate of unemployment

The natural rate of unemployment is a concept developed by economists Milton Friedman and Edmund Phelps. It refers to the minimum sustainable unemployment rate. If the unemployment gets below the natural rate, inflation tends to take off. Because there is nothing natural about unemployment, many modern economists call it the NAIRU of Non-Accelerating Inflation Rate of Unemployment.