When I was at university we dealt very little with Gross Domestic Product, or GDP. Our courses focused instead on its theoretical counterpart, Y (economic shorthand for output or income). Y is a solid foundational bedrock; GDP is not. Rather, it’s a number that changes based on how you measure it; which is regularly revised (to my annoyance when I was forecasting Swedish GDP for the Norges Bank); and which is often used by commentators to capture far more than