THE CHICAGO MERC SAYS it is closing in on its archrival, the Chicago Board of Trade. The CBOT is the world's biggest commodity futures exchange, with a first-quarter trading volume of 58 million contracts. But the Chicago Mercantile Exchange, which already bills itself as the "world's largest marketplace" because it has more trading-floor space, now boasts a boost in volume. By redoing the way it counts, the Merc reported 54.7 million contracts in the first quarter.

IN REALITY, the Merc's new volume is inflated no less than 9%. To claim bragging rights, it now includes expiring options, pork-belly deliveries, and even trades made at a related exchange in Singapore. To the CBOT, this creates an illusion of greater market liquidity. Regulators at the Commodity Futures Trading Commission say the Merc's tallying method runs counter to long-standing rules. And the Futures Industry Assn., the industry's main trade group, is subtracting the extra Merc volume from its official reports. Result: just 50.2 million first-quarter contracts for the Merc.

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