US economic history will be rewritten this week, as the most far-reaching methodological changes in years will add the equivalent of a country the size of Belgium to output in the world’s largest economy.

The most important change by the Bureau of Economic Analysis, a statistical agency of the Department of Commerce, to be announced on Wednesday, will be to start counting spending on research, development and copyrights as investment, and reflect pension deficits for the first time. Combined, they are expected to add 3 per cent to gross domestic product.

The changes – the Olympic Games of economic numbers, which take place once every five years on average – are aimed at more accurately reflecting the modern economy and will make the US the first country to adopt a new international standard.