A few years ago, Theresa May set out to make a hard-hitting speech about how too many law-abiding black men were being stopped and searched by the police. Her officials got to work on it, but soon hit a problem: the only in-depth study showed that, if anything, white men were the ones being singled out.

There was no evidence to suggest any kind of racial discrimination.

It was all a bit embarrassing, so reference to this study was removed from the draft of the speech – and the then home secretary went ahead anyway with her spirited j’accuse. Black men, she said, are up to seven times more likely to be stopped and searched. It cannot continue.

Politically, the speech was a great success. It was a potent charge, and it helped to establish Mrs May as a reformer with a social conscience.

Her figure was technically correct: when compared with the general population, young black men are far more likely to be searched. It’s just that if you look at those on the streets of an evening (as the Home Office had done), the bias vanishes.

Not that the police dared point this out: they heard and obeyed. The number of stop-and-searches conducted by police more than halved, but this raised an awkward question: what effect would this have on knife crime?