Every time I swing past Keith Hennessey’s site I am struck by how fine a job he does in explaining complex big things.

Look at his vivid account of how different graphs can be used to show supposed improvements in the US jobs market – and the real-life numbers of jobs being gained or lost.

Somewhere in the bowels of my mind I remember O-level maths about measuring the rate of change of the rate of change (ie the way a car accelerates). Yes folks, that ‘differentiation’ is back. Politicians beware:

Trivial changes in the slope of the line are not worthy of Presidential comment.  When he does comment on them, he creates a faulty impression and exposes himself to the risk that the number will again change and force him to change his message, as the Administration had to do last Friday.