Imports of vehicles may have fallen at year-end as indicated in this morning's international trade data, but vehicle inventories at dealers nevertheless rose sharply, up 1.2 percent in December on top of November's 1.8 percent build. Domestic vehicle production in the two months following earlier labor disruptions may explain the build.

Retail inventories overall rose a very strong 0.8 percent in December and together with a 0.4 percent monthly build for wholesalers helped to eke out a small positive contribution for total inventories of 0.07 percentage points to fourth-quarter GDP.

Retail inventories measure the monthly dollar value of inventories held by retailers. The advance report is released late in the month for the following month and is part of the Monthly Advance Economic Indicators report (which also includes data on wholesale inventories and international trade in goods). Final monthly data for retail inventories are released about two weeks later with the Business Sales and Inventories report.

Investors need to monitor the economy closely because it usually dictates how various types of investments will perform. The stock market likes to see healthy economic growth because that translates to higher corporate profits. The bond market prefers a slower rate of growth that won't lead to inflationary pressures. Retail inventory data give investors a chance to look below the surface of the visible economy, especially for unwanted build. If inventories are growing too fast relative to economic growth, then both production and employment will probably have to slow while those inventories are worked down.