Orders on durable goods were down almost 3.5 percent in December from a month earlier, according to a report issued Tuesday.

Figures on orders for durable goods are one way to judge the relative health of the American manufacturing sector.

Here’s some of what goes into that index:

It starts with the U.S. Census Bureau, says Dale Jorgenson, a Harvard University economics professor.

“They collect information on what the shipments were, what they actually shipped out to their customers, what they are holding in terms of inventory, and finally what they received in terms of new orders,” he says.