A key monthly index of U.S. shipping activity declined in December after rallying the month before.

The index, published by freight audit and payment firm Cass Information Systems, is based on the expenditures and shipments of Cass's clients. In December, the freight expenditure index came in at 1.528, down from 1.596 in November but up from 1.517 in October. The shipment index came in at 0.919, down from 0.947 in November and 0.921 in October.

Thomas M. Zygmunt, who manages the index for Cass, says the December figures are more the norm than the exception because shipping activity and spending generally trail off in December because store shelves are already stocked with holiday merchandise. In past years, the index has peaked in September and October only to drop off in the final two months of the year.

However, the unusually strong November data may indicate that skittish retailers were delaying orders until the last minute in order to get a clearer picture of end demand.

Zygmunt said 2009 was such an "anomaly" due to the financial crisis and severe economic downturn that it would be virtually impossible to make a prediction for 2010 based on last year's data.

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