Here is the Fed’s mea culpa on that particular topic:
Q: Why is the level of total debt outstanding in the September 18, 2015 release of the Z.1 Financial Accounts of the United States so much higher than it was in the previous Z.1 release?
Total debt outstanding was revised upwards due to methodology changes to both Treasury securities and security credit. Total debt outstanding is now the sum of two new instrument categories: debt securities (table L.208) and loans (table L.214). The aggregate of these instrument categories was previously called credit market instruments.
Treasury securities, part of the debt securities instrument category, now include nonmarketable Treasury securities held by federal government defined benefit retirement plans (FL343061145). The inclusion of federal government defined benefit retirement plans resulted in an upward revision to the level of federal government debt of about $1.408 trillion for 2014:Q4. See the published FEDS Note “Federal Government Defined Benefit Retirement Plans” for more details http://www.federalreserve.gov/econresdata/notes/feds-notes/2015/federal-….
In the domestic financial sector, borrowing previously classified as security credit liabilities (see release highlights) are now included as part of loans for the securities brokers and dealers sector. These are: (1) U.S.-chartered depository institutions loans for purchasing or carrying securities (FL763067003); (2) foreign banking offices in the U.S. loans for purchasing or carrying securities (FL753067003); and (3) Households and nonprofit organizations cash accounts at brokers and dealers (FL153067005). The revision to broker dealer debt for 2014:Q4 was roughly $962 billion.
Similarly, borrowing previously classified as security credit liabilities of the household sector are now classified as loan liabilities. Margin accounts at brokers and dealers (FL663067003) are now included in the household sector’s other loans and advances instrument category. This change resulted in an upward revision of $370 billion to the outstanding amount of household sector loans for 2014:Q4.
The bottom line:
The total revision to the level of debt outstanding (debt securities plus loans) due to these methodology changes is approximately $2.74 trillion 2014:Q4.