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A United Parcel Service truck leaves the yard October 29, 2010 in Chicago, Ill. Why do UPS earnings matter as a signal for the rest of the economy? - 

Adriene Hill: Later this morning, UPS will tell us how it did for the last few months of 2011. The shipper's earnings report gets a little more attention than others because it's a bellwether. Some economists say it points the way to what lies ahead for the

Marketplace's Nancy Marshall-Genzer tells us why.

Nancy Marshall-Genzer: If Big Brown is making a lot of green, the economy is headed up. At least that's what economists say. UPS is a bellwether because when the economy improves, companies make and sell more stuff. And somebody has to ship it. UPS was very busy during the last quarter of 2011.

Mark Zandi is chief economist at Moody's Analytics.

Mark Zandi: When there's a lot of economic activity, when consumers are out buying things, they're going to benefit from that. The better the economy, the more things move and that helps companies like UPS.

And other shippers like FedEx, which economists also follow. But shipping isn't the only way to take the economy's pulse. Caterpillar sheds light on construction. John Deere helps economists track agriculture. And Apple is a core indicator for the tech industry.

In Washington, I'm Nancy Marshall-Genzer for Marketplace.