WHEN unveiling Britain's annual budget on March 19th George Osborne, the chancellor of the exchequer, crowed that the British economy was forecast to grow at an annualised rate of 2.7% in the first quarter of 2014, the fastest in the rich world. His critics countered that whereas output in America and Germany has already topped the pre-crisis peak, Britain’s will not get there until later this year. The data-point at issue in both cases is gross domestic product, or GDP, the total value of all goods and services produced within an economy each year. GDP is of critical economic importance; thousands of economists use estimates of the total amount spent or (equivalently) earned each year in their research. Governments also rely heavily on the figure, to shape policy or determine how much public spending is affordable. Yet GDP seems an impossibly complex thing to measure in a modern economy. How do countries calculate it?
