A changeover to a market-consistent valuation of the police and fire services' pensions has added £15 billion to the UK government's liabilities, as part of a total £70 billion increase announced by the Treasury.
The method for calculating the liabilities of the police and fire service was changed in the financial year 2004/2005 from the reserve accounting framework to the market-consistent FRS 17 standard that is applied to private sector companies.
The change over to a discount rate pegged to the level of AA rated corporate bonds resulted in the real discount rate falling from 3.5% to 2.4%.
This was - according to Andrew Johnson, deputy actuary of the Government Actuary's Department - an example of how: "Small changes can have a big impact."
A bigger change than the move over to realistic balance sheet reporting was the restatement of interest payable, as pensions moved closer to payout over the financial year - a move that added another £44 billion to the total of liabilities.
A combination of the reassessment of poor quality data, increases in longevity assumptions and large pay increases in the health sector was responsible for the remainder of the increase.