With interest rates at their lowest point in years and the volatility in the stock market, it has become nearly impossible to predict whether most retirement accounts will earn enough to fund a 4 percent withdrawal rate.

Like everyone else who dines at a restaurant in Omaha, Nebraska, Buffett is charged a 2.5 percent tax thats used in part to defray $850 million in unfunded retirement expenses for city workers.

While the rates arent directly followed by most investors, even small movements can affect the value of what Morningstar Inc. estimates is about $3.6 trillion in funds including pension and savings accounts that track global indexes.

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