From Tim Hartford:

Unexpected definitions can affect targets as well as trends. In the UK, the most notorious target is the one that keeps being missed: a promise to keep net migration under 100,000. In 2010 the then prime minister David Cameron challenged voters to kick him out if he missed the target. He did, and in a way, so did they. Encouraged by six years of failure to hit the target as home secretary, Theresa May has, now as prime minister, renewed the promise again.

How is this to be achieved? Leaving the EU won’t do the job alone: net immigration from outside the bloc has consistently exceeded 100,000. So attention has turned to a policy that many people regard as obvious: keep low-skilled immigrants out, and prioritise the highly skilled. For example, a recent policy paper published by the lobby group “Leave Means Leave” calls for a “moratorium on unskilled visas”. The paper proposes that working visas should be issued only to those who meet certain requirements, including a job offer on a salary of at least £35,000.

But this is an interesting slippage in the use of the word “unskilled”. About three quarters of UK employees earn less than £35,000, and as Jonathan Portes of King’s College London points out, the majority of nurses, primary schoolteachers, technicians, paralegals and chemists earn less than this figure.

Proposing an end to “unskilled migration” sounds reasonable to many voters; they might find it less reasonable if they realised that some definitions of “unskilled” would exclude a teacher or an intensive care nurse.