Should you happen to spot me these days lurking outside a Calvin Klein boutique, notebook in hand, I assure you I have a serious purpose. I’m applying the method of the former US Federal Reserve chairman Alan Greenspan, who relished statistical minutiae and believed that sales of men’s underpants – an item so out of sight that a chap could readily choose not to replace worn-out ones when he senses an economic pinch ahead – offer a reliable indicator of impending downturns.