WHEN JIMMY Carter, the Democratic candidate for American president in 1976, wanted to criticise the record of the incumbent Gerald Ford, he reached for a number invented by the economist Arthur Okun. A rough-and-ready indicator of the state of the economy, what Okun called the economic discomfort index added together the unemployment rate with the level of inflation. Four years later Ronald Reagan, the Republican candidate, renamed the indicator to the pithier misery index and used it against Mr Carter, who had presided over rising inflation and unemployment. Reagan went on to win the election and the subsequent one, in 1984, as the index fell on his watch.