While a new index tracking diesel fuel purchases by truckers fell dramatically from December to January, the fall is most likely due to the normal seasonality of freight

, said Bob Costello, chief economist for the American Trucking Associations. "I would expect it to fall from December to January," he said. "That's not a surprise."

According to Costello, freight demand typically drops off in January following the holidays, as people just don't need as much freight during the month. "It's the nature of the freight industry."

The new indicator is called the Ceridian-UCLA Pulse of Commerce Index (PCI), which is based on real-time diesel fuel purchases using a Ceridian card by over-the-road truckers at more than 7,000 locations across the U.S. The index is issued monthly and captures the real-time monitoring of the movement of raw materials, goods-in-process and finished goods to U.S. factories, retailers and consumers.

In January, the index fell at the annualized rate of 36.8 percent after gaining 60.8 percent in December.

But Edward Leamer, director of the UCLA Anderson Forecast and chief economist for the Ceridian-UCLA Pulse of Commerce Index, said we should be looking for a more accurate picture from the three-month moving average for January, which showed a 3.3 percent gain at an annualized rate, compared with an increase of 14.6 percent in the previous month. The three-month moving average, Leamer said, provides a smoother picture of what's going on.

Leamer said there's also a lot of month-to-month variability that could contribute to changes in the index, such as the number of workdays in a month and weather conditions. For example, the number of net workdays in December, excluding holidays, was 23, while January only had 21 net workdays.

In addition, when you look at January 2009, the index is actually 3.6 percent above its prior year level. In the most recent Ceridian-UCLA Pulse of Commerce Index report, it says this value is similar to pre-recession year-over-year values.

According to Leamer, a lot of businesses didn't stock up in months prior to the holidays. However, when they started doing better than expected, they ended up having to stock up in larger quantities than usual in December right before the holidays. This also may have contributed to the low January numbers.

Costello said that with any index, it's imperative to take seasonality into account to provide the most accurate picture. And the new index does offer seasonally-adjusted figures, which Leamer calls "off the shelf." The researchers use a standardized seasonal adjustment model, one which Leamer says may not work for the trucking industry. The annualized index figures discussed earlier, including January's 36.8 percent dip, is seasonally adjusted.

When looking at the three-month moving average on a seasonally-adjusted basis, the index was up 7.3 percent in December, versus a gain of 9.6 percent in January.

For more information on the Ceridian-UCLA Pulse of Commerce Index, visit www.ceridianindex.com.