WHEN JIMMY Carter, the Democratic candidate for American president in 1976, needed to criticise the file of the incumbent Gerald Ford, he reached for a quantity invented by the economist Arthur Okun. A rough-and-ready indicator of the state of the financial system, what Okun known as the financial discomfort index added collectively the unemployment charge with the extent of inflation. 4 years later Ronald Reagan, the Republican candidate, renamed the indicator to the pithier distress index and used it in opposition to Mr Carter, who had presided over rising inflation and unemployment. Reagan went on to win the election and the next one, in 1984, because the index fell on his watch.