Tracking current levels of debt is a complex but rather straightforward process. Making future projections is much more difficult for a number of reasons. For example,
before the 9/11 attacks the Bush Administration projected that there would be a $1.288 trillion surplus from 2001 through 2004 in the 2002 U.S. Budget. In the 2005 Mid-Session Review, however, this had changed to a projected deficit of $850 billion, a swing of $2.138 trillion. Table 7 in this latter do ent states that 49% of this swing was due to "economic and technical re-estimates", 29% was due to "tax relief", and the remaining 22% was due to "war, homeland, and other enacted legislation". Reasons for the inaccuracy of future projections include economic growth being different than projected and changes in fiscal policy (tax cut and War on Terror).
In extra, projections between different groups will sometimes differ because they make different assumptions. For example, an August 2003 CBO do ent projected a $1.4 trillion deficit from 2004 through 2013. However, a joint analysis put out by the Center on Budget and Policy Priorities, the Committee for Economic Development, and the Concord Coalition a month later stated that "In projecting deficits, CBO follows mechanical 'baseline' rules that do not allow it to account for the costs of any prospective tax or en lement legislation, no matter how likely the enactment of such legislation may be". The analysis added in a proposed tax cut extension, AMT relief, prescription drug plan, and increases in defense, homeland security, international, and domestic spending. This raised the projected deficit from $1.4 trillion to $5.0 trillion. Hence, the assumptions on which the projections are based are also very important.
Despite the drawbacks of making future projections, however, a responsible government must arguably make long-run projections so it can prepare the country for future possibilities. The federal government does provide long-run budget projection in Table 13-2 on page 209 of the Analytical Perspectives of the 2006 U.S. Budget. It projects that the federal debt held by the public will reach 249 percent of GDP in 2075. This is more than double the maximum reached during World War II and nearly four times its current level. Most of this increase is due to projected increases in en lement spending and the resulting interest on the debt. It is worth noting that this is a projection, not a prediction. This projection assumes normal economic conditions and that government policies will follow current law. The stress of a quadrupling of the debt would likely cause one or both of these items to change.