The fiscal policies to fight the great recession in the United States were not administered in a way that would allow us to assess this a natural experiment. However, there have been many situations in the past where there was such a natural-experiment setting, and that allows us to approximate the effect of these fiscal policies. For example, if a dollar of additional government spending, when such variation is experimental in nature creates two dollars in private consumption and investment (a multiplier of two), then when the government spends a trillion dollars in a way that may be endogenous, you could guess that the effect on private consumption would be something like two trillion.
The CBO used this methodology in their report Estimated Impact of the American Recovery and Reinvestment Act on Employment and Economic Output from October 2010 Through December 2010 (2011):
The American Recovery and Reinvestment Act of 2009 (ARRA) contains
provisions that are intended to boost economic activity and employment
in the United States. Section 1512(e) of the law requires the
Congressional Budget Office (CBO) to comment on reports filed by
recipients of ARRA funding that detail the number of jobs funded
through their activities. This CBO report fulfills that requirement.
It also provides CBO's estimates of ARRA's overall impact on
employment and economic output in the fourth quarter of calendar year
2010. Those estimates—which CBO considers more comprehensive than the recipients' reports—are based on evidence from similar policies
enacted in the past and on the results of various economic models.
On that basis, CBO estimates that ARRA’s policies had the following
effects in the fourth quarter of calendar year 2010:
- They raised real (inflation-adjusted) gross domestic product (GDP)
by between 1.1 percent and 3.5 percent,
- Lowered the unemployment rate by between 0.7 percentage points and
1.9 percentage points, Increased the number of people employed by between 1.3 million and 3.5 million, and
- Increased the number of full-time-equivalent jobs by 1.8 million to
5.0 million compared with what would have occurred otherwise, as shown in Table 1. (Increases in FTE jobs include shifts from
part-time to full-time work or overtime and are thus generally
larger than increases in the number of employed workers.)