[...] the question has arisen of whether the Fed and the ECB should recognize that 2 percent inflation in a global economy awash in cheap oil, available workers and money-saving technologies might be too ambitious and should lower their targets. The major central banks' historically low interest rates and mass bond purchases have failed to offset the pressures of lower energy prices and tepid consumer demand. The credibility of the Fed and ECB hinges in part on their ability to achieve that level of inflation. [...] as that target remains distant, central bankers must maintain the extraordinary interventions they began with the 2008 global financial meltdown. Mohamed El-Erian, chief economic adviser at Allianz and a former deputy director of the International Monetary Fund, says the dilemma stems from an overreliance on interest-rate policies to support a floundering Europe. "When the world economy is fragile, you end up restraining demand," says Stephen Oliner, a former Fed economist who is now a resident scholar at the American Enterprise Institute, a conservative think tank.