Exports plus Imports divided by GDP is the total trade as a percentage of GDP. More information is available at http://pwt.econ.upenn.edu/Documentation/append61.pdf
For proper citation, see http://pwt.econ.upenn.edu/php_site/pwt_index.php
Source Indicator: openc
Source: University of Pennsylvania
Release: Penn World Table 7.1
University of Pennsylvania, Openness at Current Prices for United States [OPENCPUSA156NUPN], retrieved from FRED, Federal Reserve Bank of St. Louis https://research.stlouisfed.org/fred2/series/OPENCPUSA156NUPN/, November 25, 2015.