Everything you need to know about the Congressional Budget Office

— -- The Congressional Budget Office will release its first analysis of the Republican Senate health care bill Monday, again shining a spotlight on the nonpartisan reviewers of congressional legislation.

As the House of Representatives worked its way through its version of a health care law, Democrats frequently pointed to the office's estimates -- or lack thereof, in some cases -- as rationale to vote against or delay movement on the Republicans' plans.

“It is reckless for Republicans to make Congress vote on this mess of a plan before we have those answers from CBO,” Senate Minority Leader Chuck Schumer, D-N.Y., argued in early March as two House committees reviewed the bill and then voted along party lines to approve it.

The CBO eventually produced analyses of three drafts of the American Health Care Act that each showed a jump in uninsured Americans and increasing premiums for some along with a deficit reduction.

The White House and supporters of the bill criticized the CBO's accuracy after it released its first analysis. In March, White House press secretary Sean Spicer leveled stinging criticism against the CBO, which has analyzed and predicted the financial impact of legislation for more than four decades.

"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 correct on employer-sponsored coverage and an overall surge in coverage.

The CBO has acknowledged the challenge of accurately forecasting the future impact of legislation, but says it strives to provide transparent analysis without party allegiance. The Senate requires the CBO to produce a report before it considers the bill.

Here's a look at what you need to know about the CBO scoring:

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 nonappropriations bill approved by a full House or Senate committee.

In addition to objectivity, the office seeks 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 earlier in the year 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 that number ultimately came to 12 million.

Former CBO Director Doug Elmendorf told The Associated Press that it was "difficult" to be accurate when predicting the effects of major policy efforts but maintained that the "CBO's projections for the ACA in 2010 were much more accurate than many Republican opponents of the law." He specifically noted that the office was correct in its analyses of employer-sponsored coverage and claim there would be an overall surge in coverage.

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."

ABC News' Ryan Struyk contributed to this report.