The argy-bargy over whether or not the 'burden of taxation' in this country has gone up or down under this government has had the interesting side effect of drawing attention to what is a pretty murky area, namely the Treasury's accounting conventions.

The immediate debate turns largely on whether or not the working family's tax credit is seen as an increase in expenditure or as a reduction in tax. If it is the former, which is what the conventions require, then the tax burden has gone up, but if it is the latter then it has not.

In this case the figures and differences are relatively small in all senses except political. But there are other areas where changes in treatments of expenditures and taxes have been made, mostly rather quietly, by this government which cumulatively have quite a substantial, unrecorded and unquantified effect. There are four in particular.

First, there are the netting-off taxes and expenditures, apparently reducing both the burden of taxation and the burden of expenditure. The WFTC is an example, but so was the windfall tax, and we wait to see if the £300m from increased tobacco duty, apparently to be allocated to the health service, and other promised hypothecations, will also be 'lost' in this way.

Second, there is the private finance initiative when items which should be seen as today's expenditure are actually spread over time in the future, incidentally at a greater cost to the taxpayer, thus reducing public expenditure today.

Third, there is the simple raiding of the National Lottery - itself a gigantic voluntary tax - for things that should come out of ordinary public expenditure. And fourth, there are guarantees given in respect of public works that should be scored against public expenditure, but are not now scored. The effect of these is to improve not just politically sensitive ratios of tax and expenditure but to magnify Gordon Brown's so-called war chest. The new treatments may be right. But there are conventions, and these have to be observed if we are to get a proper feel for the public finances over time, in the same way that private sector accounting conventions allow us to understand how companies are doing over time.

Here the Treasury fails. Not only do politicians move the goal posts to their advantage, they then don't explain clearly that they are doing so, and they don't explain the effect. And then they bluster and whinge when they're challenged. It's not good enough.

Sir Peter Kemp is chief executive of the Foundation for Accountancy and Financial Management