In early 2009, Christina Romer, who was then the chair of the Council of Economic Advisers, and Jared Bernstein, who was then an economic adviser to Vice President Joseph Biden, forecast how long they expected it would take for real GDP to return to potential GDP, assuming that Congress passed fiscal policy legislation proposed by President Obama:
It should be understood that all of the estimates presented in this memo are subject to significant margins of error. There is the obvious uncertainty that comes from modeling a hypothetical package rather than the final legislation passed by the Congress. But there is the more fundamental uncertainty that comes with any estimate of the effects of a program. Our estimates of economic relationships ... are derived from historical experience and so will not apply exactly in any given episode. Furthermore, the uncertainty is surely higher than normal now because the current recession is unusual both in its fundamental causes and its severity.
Why would the causes of a recession and its severity affect the accuracy of forecasts of when the economy would return to potential GDP?