Members of Congress in both political parties depend upon economic projections from the Congressional Budget Office (CBO) when shaping legislation, but that agency’s analyses often miss the mark by wide margins, according to a new analysis by a nonprofit government transparency watchdog.

“While the agency repeatedly states that it is ‘objective’ and ‘nonpartisan’ throughout its promotional materials, it has come under fire for underestimating the costs of tax-and-spend policies and favored Democratic initiatives and overestimating economic losses from Republican-led initiatives like tax reductions. CBO’s routine forecasting has been generally accurate, but even self-audits find they’ve been wildly incorrect on major pieces of policy and ideologically significant initiatives,” according to the report from Open the Books (OTB).

“CBO has audited its predictions every year since 2017. A 2026 report found that from 1983 to 2024 (the earliest and most recent years available), the average absolute error in revenue projections was 6 percent, with overestimations and underestimations occurring at about the same rate,” OTB explains.

An analysis by The Washington Stand of the annual over- and under-estimations by CBO of federal revenues, outlays, and impact on the annual budget deficit for the eight years between 2017 and 2025 found an 8.2% average for the five years in which CBO underestimated revenues collected by the Department of the Treasury and a 4% average for the three years of overestimations.

On CBO projections for annual outlays, or spending, there were five years in which the agency’s projection underestimated the actual amount by an average 3.42%. The three years with over-estimations averaged 2.43%. The projections for the impact on the federal government’s annual budget deficit included three years of underestimates an average of 1.7% of Gross Domestic Product (GDP). For the five years of over-estimation, the average came to 1.9% of GDP.

The OTB evaluation pointed to a pattern in which CBO projections were generally in the same range as those from the Office of Management and Budget (OMB) in the White House, but the congressional forecasters’ projections were repeatedly and often substantially off on major legislation.

• “Despite extensively working with legislators on the Affordable Care Act, the CBO scored the ACA’s cost at $788 billion net cost of coverage provisions between 2010 and 2019 for Obamacare. By 2012, the new estimate for the period of 2012-2022 was $1.76 trillion. That’s $972 billion or a 123-percent higher cost for a decade of policy.

• “Within the American Rescue Plan Act of 2021, the CBO predicted the highly expanded Child Tax Credit would decrease revenues by $21 billion and increase outlays by $88 billion, at a cost of about $109 billion. However, when calculating the possibility of extending the credit, its new cost, according to CBO, suddenly spiked to $185 billion over the budget window (a 70% increase).

• “The Inflation Reduction Act of 2022 was, despite its name, packed with green-energy investments worth over $400 billion. CBO calculated the whole bill would create savings and revenue worth $738 billion, and reduce the deficit by $238 billion over 10 years. In 2024, those predictions were revised to increase the deficit by $300 billion.

• “When the American Recovery and Reinvestment Act (ARRA) was being considered, the CBO and the staff of the Joint Committee on Taxation estimated that it would increase budget deficits by $787 billion between fiscal years 2009 and 2019. By 2012, the CBO changed its estimate for that period to $831 billion, then again to nearly $840 billion in 2015. [But] In a February 2025 report, CBO said, ‘The effects of ARRA on output peaked in the first half of 2010 and have since diminished.’”

• Another part of the 2009 stimulus package was adding provisions for unemployment compensation. In February 2009, CBO predicted a $25 weekly unemployment insurance bonus would cost $39.2 billion over the 10-year budget window. But a revised score by CBO six years later showed the costs would be $64 billion — or 63% more than predicted.

The OTB report also pointed to a major lack of transparency regarding the CBO, which is likely a function of the fact that Congress exempted itself, including its agencies like CBO, from coverage under the Freedom of Information Act (FOIA), which is the main law providing for individual citizens’ access to most federal documents.

As a result, while the number of staffers employed by CBO is known, as is how many of them work on specific issue areas, their individual compensation costs are not available to the public. Salaries for the 12,000-plus congressional staffers working for senators, representatives, and committees is public.

The OTB report also noted that the major legislative bills on which the agency’s forecast was created to be a nonpartisan, unbiased source of economic prediction-making. It has since evolved to have a losing track record, while becoming increasingly enmeshed in the legislative process. “The agency suffers from a lack of transparency on staff and salary data, while producing reports on ideologically charged issues like health care and climate change which influences billions of dollars of spending,” OTB observed.

A CBO spokesman declined to comment on the OTB report but noted that the congressional agency “publishes information about its estimating methods and the accuracy of its projections here” and also has “materials describing our analytical approach to health care policy here and here.”