"Given how ubiquitous and central the CBO has become to government policy, it's surprising how recently it was created [in 1974]....The CBO is responsible for analyzing the effects of spending and of commitments to spend on the long-term federal budget. As the federal government began to rely on deficit spending in the 1980s, the Congressional Budget Office was anointed the official keeper of budget statistics, along with the OMB. This was formalized in 1985, when the rising public anxiety about mounting deficits -- a foreshadowing of much higher levels of concern after 2008 --le to the passage of a bipartisan bill to keep the deficit in check.
"...One of the core functions of the CBO is to estimate how much the government will spend on health care and Social Security. Though the CBO is staffed by nonpartisan economists and accountants, its analysis is based almost entirely on guesstimates about the future rate of inflation, the future rate of growth, and the future rate of employment -- all of which are inherently unknowable and each of which determines how much revenue the government will collect in the form of taxes."
Zachary Karabell, THE LEADING INDICATORS (2014) pp. 152-53.