A key monthly index of U.S. shipping activity turned down in October after rising in September. But the keeper of the index said October's results do not necessarily foreshadow a weakening of freight volumes.

The index, published by freight audit and payment firm Cass Information Systems, is based on the expenditures and shipments of Cass's clients. In October, the freight expenditure index came in at 1.52, down from 1.55 in September but up from 1.47 in August. The shipment index came in at 0.92, down from 0.97 in September and 0.94 in August.

Thomas M. Zygmunt, who manages the index for Cass, said the decline might be more a reflection of the past than a window on the future. The October shipment index has dropped below September's results for the past three years. In fact, Zygmunt said, he wouldn't be surprised if the index shows declines in November and December, as that has also been the case for the last three years.

Since the start of 2009, the Cass index has fluctuated wildly. Shipment activity in February rose over January levels. The index then declined in March and April, increased in May and June, and declined in July, only to rise again in the next two months.

In a sobering reminder of how volumes have declined since 2006 and 2007, the shipment index stayed consistently in the 1.2 to 1.3 ranges during those two years, Zygmunt said. The index has not exceeded 1.0 since November 2008.

In 2008, Cass audited about 26 million shipments representing roughly $17.5 billion in shipper expenditures.