Manzi thinks it's unknowable:

First, we should treat anybody who states definitively that the result of stimulus policy X will be economic outcome Y with extreme skepticism. And weaseling about the magnitude of the predicted impact such that all outcomes within the purported range of uncertainty still magically lead to the same policy conclusion doesn’t count; we should recognize that we don’t even know at the most basic level whether stimulus works or not. Second, “boldness” in the face of ignorance should not be seen in heroic terms. It is a desperate move taken only when other options are exhausted, and with our eyes open to the fact that we are taking a wild risk. ]]>