A possible trade agreement between the UK and the US has become a controversial issue in the UK elections. The Labour Party alleges that the UK’s National Health Service (NHS) is on the table in negotiations. (In Northern Ireland, at least, the NHS seems to be on the floor, and in general is delivering no more than average results in European terms. Nevertheless it is much prized in the UK) . For the Conservatives, a trade agreement with the US is a big priority after Brexit. The US Secretary of State, Mr Pompeo, had earlier said that this will also be a priority for the US. The then US National Security Advisor, Mr Bolton, suggested that there could be a series of partial agreements on individual sectors. But this was a bad idea for lots of reasons. Let’s take just three of them. Firstly, no US-UK agreement, even partial, can be finalized until the nature of the future UK- EU relationship is clear, and those negotiations have not yet begun. Secondly, the tendency is always in modern economies for sectors to become more and more interrelated, and so it’s harder and harder to treat them separately. Thirdly, statisticians will recognise the related question of ” degrees of freedom”: making a number of partial agreements ends up by putting an impossible burden of adjustment on the final sector to be negotiated.