CBO Report: Estimated Impact of the American Recovery and Reinvestment Act on Employment and Economic Output as of September 2009 [open pdf - 636KB]
"CBO's [Congressional Budget Office] current estimates do not reflect any change in the agency's assessment of the effect that each dollar of spending increase or revenue decrease has on output and employment. Since March, CBO has continued to examine new research on the relationships between changes in government policy and changes in output and employment. To date, that examination has generated no significant change in CBO's assessment of those relationships. CBO has also examined incoming data on output and employment during the period since ARRA's [American Recovery and Reinvestment Act] enactment. However, those data are not as helpful in determining ARRA's economic effects as might be supposed, because isolating the effects would require knowing what path the economy would have taken in the absence of the law. Because that path cannot be observed, the new data add only limited information about ARRA's impact. Economic output and employment in the spring and summer of 2009 were lower than CBO had projected at the beginning of the year. But in CBO's judgment, that outcome reflects greater-than-projected weakness in the underlying economy rather than lower-than-expected effects of ARRA."
Congressional Budget Office: http://www.cbo.gov/