Commodities traders who buy and sell as much as $5.67 trillion of raw materials a year say the benchmark prices for everything from oil to iron ore to gasoline are wrong as often as 27 percent of the time.

In a Bloomberg News survey conducted during the past eight weeks, 85 traders and analysts said they have little confidence in the assessed prices of crude, metals and iron ore. Regulators, including European Union Competition Commissioner Joaquin Almunia, may examine commodities markets, having already increased investigations of manipulation of benchmarks for interest rates, derivatives, foreign exchange and oil.

Five years after the global credit crisis prompted more regulation of banks, benchmark prices for hundreds of commodities are determined through surveys of anonymous traders who may have a stake in the outcome of the assessments. Unlike stock prices, available in real time at regulated exchanges for all investors to see, many raw materials that go into food, clothing and power are bought and sold in private.