A long time ago, back when I was at school, I was given some rather good advice about putting forward an argument: always state your assumptions. Now, I'm not sure where the current crop of junior Home Office Ministers went to school, and I'm loth to point fingers at his former employer, but it looks as if Liam Byrne didn't learn this lesson, or promptly forgot it when he entered politics.

For Liam has seen the polling on the cost of ID cards and has come up with a whizzo scheme for reducing the cost from the likely £300 a skull: let's just glue together the existing data the Government holds.

Now at one level, this shows the kind of practical thinking that the Home Office has been rather poor at lately. But when you dig into it just wee bit, it shows that someone has made a huge assumption:

All Government Held Data is Clean. Or at least: clean enough to depend on for a critical application that requires super clean data to be at all effective.

Which shows not least that Liam has not been around any large consumer/citizen database, let alone a government one which has built-in enhanced probabilities of data-subject attempts to game the system. Starting from the existing databases and trying to build a system of foreign keys without a solid, reliable master index (let's call it the National Identity Register for a giggle) to start with is so cart before horse that it's a joke. It's the very definition of 'a house built on sand'.