ABC News(WASHINGTON) — White House press secretary Sean Spicer leveled stinging criticism against the nonpartisan Congressional Budget office Wednesday, assailing its accuracy amid Democratic complaints that the agency would not have an opportunity to review the new health care legislation before a vote.

“If you’re looking to the CBO for accuracy, you’re looking in the wrong place,” said Spicer. “They were way, way off last time in every aspect of how they scored and projected Obamacare.”

Spicer was right, in part: The office predicted millions more people would enroll in health exchanges than did, but the CBO maintains it was right on employer-sponsored coverage and an overall surge in coverage.

The introduction of the Republican-backed health care bill intended to replace the 2010 Affordable Care Act and Spicer’s comments on the CBO’s forecasting have cast a spotlight on the office, which has analyzed and predicted the financial impact of legislation for more than four decades.

The office itself notes the challenge of producing entirely prescient predictions, but strives to provide transparent reports without party allegiance. Here’s a look at what you need to know about the CBO:

History

The CBO was created as a part of the Congressional Budget and Impoundment Control Act of 1974, which set standard practices in Congress for the development of the federal budget and also established budget committees in the House of Representatives and Senate.

The office was explicitly established as a nonpartisan body. The act states, “All personnel of the [CBO] shall be appointed without regard to political affiliation and solely on the basis of their fitness to perform their duties.” Further, the CBO does not make recommendations and should avoid “value judgments.”

Processes

As part of its responsibilities, the CBO gathers information from executive and legislative branch departments and agencies — which are required to provide the office with the data they seek — to develop estimates of the effects of congressional action. The CBO provides direct assistance to the Budget, Ways and Means, Appropriations and Finance committees, but its reports can be requested by any other committee or member of Congress.

The CBO says its “economists and budget analysts produce dozens of reports and hundreds of cost estimates for proposed legislation,” per year. Some of its regular work includes economic projections, analysis of the president’s budget and sequestration reports, as well as cost estimates of every non-appropriations bill approved by a full House or Senate committee.

In addition to objectivity, the office seeks for its analyses to be as transparent as possible, publishing its methodology with each report. The CBO website explains that each analysis it produces is based on a number of factors, including “federal programs and the tax code,” “relevant research literature,” “data collected and reported by the government’s statistical agencies and by private organizations” and “consultation with numerous outside experts.”

Accuracy

Spicer’s claims Wednesday brought increased attention to the CBO’s projections during the last health care battle. The office forecast that in 2016, 23 million people would be enrolled in health care exchanges, but ultimately there were 12 million.

The office notes that frequent changes to legislation after its projections make assessments of their accuracy precarious, but as part of its commitment to transparency, it tries “to communicate to the Congress the uncertainty of the agency’s estimates.”

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