This Wikipedia table lists public debt figures as a percentage of GDP, using numbers from the CIA World Factbook and OECD and IMF publications.
The disparity between sources is, well, pretty astounding.
Here are the numbers for the UK, for example:
CIA '08 47.20
OECD '08 56.8
IMF '08 43.4
And now, the US:
CIA '08 39.7
OECD '08 70.0
IMF '08 61.5
So, what’s the deal? Is there that much disagreement in methods of calculating national debt - or is the disparity in calculations of GDP?
No, the disparity appears to be in the author’s ability to distinguish between, say, central government debt and general government debt. The CIA actually estimates the latter to be 60.8% of GDP, comparable to the other two. According to the OECD website, US central government debt is 40.0% of GDP, comparable to the CIA estimate. I really can’t tell what IMF is up to, as they pull data from many sources, and, of course, the Secretary of State denies any knowledge of their actions.
One key stumbling block is distinguishing between “All debt issued by the government including debt held in governmental accounts (such as the Social Security Trust Fund, revolving funds, and special funds)” and “Debt held by the public”.
Debt held by the public includes all federal debt held by individuals, corporations, state or local governments, foreign governments, and other entities outside the United States Government…
More: http://www.treasurydirect.gov/govt/resources/faq/faq_publicdebt.htm