According to Paul Krugman, the complex multiplier for the United States is about equal to 2.
The American Recovery and Reinvestment Act of 2009 earmarked $787B in deficit spending, of which $330.4B was spent in 2009. (Total government deficits for 2009 were $1251.7B.) The increase in nominal GDP from 2009 to 2010 was $541.2B (3.8%).
If we assume that this growth resulted from the ARRA stimulus package, what was the implied multiplier associated with the stimulus package?