The United States is the world's largest national economy, representing 22% of nominal global GDP and 17% of global GDP (PPP). The United States' GDP was estimated to be $17.914 trillion as of Q2 2015. The U.S. dollar is the currency most used in international transactions and is the world's foremost reserve currency. Several countries use it as their official currency, and in many others it is the de facto currency. The United States has a mixed economy and has maintained a stable overall GDP growth rate, a moderate unemployment rate, and high levels of research and capital investment. Its seven largest trading partners are Canada, China, Mexico, Japan, Germany, South Korea, and the United Kingdom.