The natural unemployment rate, sometimes called the structural unemployment rate, was developed by Friedman and Phelps in the 1960s. It represents the hypothetical unemployment rate that is consistent with aggregate production being at a long-run level. The natural rate of unemployment is a combination of structural and frictional unemployment. It is present in an efficient and expanding economy when labor and resource markets are at equilibrium. The natural unemployment rate occurs within an economy when disturbances are not present.