When an IT scheme costs too much to declare openly, officials sometimes present to Parliament the partial costs only.
So when an MP asked for the costs of the Defence Information Infrastructure he was told £2.3bn. He was not told that this estimate was the first phase only. The scheme in fact was projected to cost £7bn, which Parliament discovered only years later.
Similarly the NHS IT scheme was announced as costing £2.3bn in 2002. Internally, the Department of Health had costed the scheme at £5bn but kept the evidence of the estimate hidden. Now it's due to cost at least £12.7bn.
With ID Cards it's a bit different. When the virtuosos of spin want to make the costs of a project seem small they tend to express the costs as annual amounts or over a period of say three years.
When they want to make the financial benefits of a scheme seem large, they gather in a heap the savings over a long period of say 10 years.
The Home Office has gone way beyond this in a report which it has published today.
The Home Secretary Jacqui Smith, the Home Office and the Identity and Passport Office have expressed the benefits of ID Cards over 30 years.
A week is a long time in politics. How many of today's MPs will be alive in 2039, let alone remember a promise made by a Home Office minister in 2009?
For the record here's the claim made in the Home Office's press release of today, 6 May 2009:
"An impact assessment of the National Identity Scheme published today estimates an economic benefit of up to £6bn over the next 30 years."
Professor Ross Anderson, Professor of Security Engineering at Cambridge University's Computer Laboratory, said the government's estimates today on the costs of ID Cards have been underestimated "by an order of magnitude".
PS: Jacqui Smith is to lose her job as Home Secretary within weeks, says today's Sun. This puts the 30-year promise into context.