From a Congressional Budget Office estimate released today on the impact of some amendment or other to the Senate stimulus bill:
The macroeconomic impacts of any economic stimulus program are very uncertain. Economic theories differ in their predictions about the effectiveness of stimulus. Furthermore, large fiscal stimulus is rarely attempted, so it is difficult to distinguish among alternative estimates of how large the macroeconomic effects would be. For those reasons, some economists remain skeptical that there would be any significant effects, while others expect very large ones.
It’s sort of like that mutual fund boilerplate, “Past performance is no guarantee of future results.” Except that we’re not even sure of what the past performance was. (And I say this as somebody who thinks the stimulus legislation is on balance a good idea.)