This is University of Chicago economics professor Casey B. Mulligan writing at The New York Times economics blog:
I have illustrated the Keynesian-multiplier-1.5 as a red line in the chart below, and the actual results as blue squares. The blue square at the end of the red line is the data for the fourth quarter of 2009. If the multiplier of 1.5 held up, all of the data for the subsequent quarters should have appeared on the red line. (The quarters represented by the squares are not in chronological order.)
Instead, actual G.D.P. growth ended up below the red line and, more important, the quarters with more government spending growth tend to be those with less G.D.P. growth.
The blue squares showing actual results for our economy do not fit anywhere in the cone formed by the two Keynesian hypotheticals, suggesting that, contrary to the Keynesian promises, the stimulus law did not noticeably increase G.D.P. and might even decrease it.