From "Second Helpings" by James Surowiecki:
But, by any reasonable measure, the $800-billion stimulus package that Congress passed in the winter of 2009 was a clear, if limited, success. The Congressional Budget Office estimates that it reduced unemployment by somewhere between 0.8 and 1.7 per cent in recent months. Economists at various Wall Street houses suggest that it boosted G.D.P. by more than two per cent. And a recent study by Mark Zandi and Alan Blinder, economists from, respectively, Moody’s and Princeton, argues that, in the absence of the stimulus, unemployment would have risen above eleven per cent and that G.D.P. would have been almost half a trillion dollars lower. The weight of the evidence suggests that fiscal policy softened the impact of the recession, boosting demand, creating jobs, and helping the economy start growing again.